An opportunistic study was carried out in central Mexico, where one of the world's largest wastewater reuse schemes for agricultural production is located. This scheme provided a unique opportunity to assess the health impact of exposure to wastewater of different qualities on intestinal parasitic infections and diarrhoeal diseases. The central objective of the study was to evaluate the effect of hydraulic retention on reducing the health risks associated with wastewater use. Exposure groups were defined according to eligibility procedures and to the quality of irrigation water. Microbiological quality was measured using nematode eggs and faecal coliforms as indicators. The exposure groups involved households: a) exposed to untreated wastewater; b) exposed to wastewater retained in a single reservoir; c) exposed to wastewater which had passed through two reservoirs in series, and been retained for some time in both; and d) nonwastewater-exposed households (controls). The study outcomes included risk of Ascaris lumbricoides, Entamoeba histolytica and Giardia lamblia infections, as well as the risk of diarrhoeal diseases. The study design was based on two cross-sectional surveys (rainy and dry seasons), and the analyses focused on both comparison of risks between the different exposure groups as well as the identification of at-risk groups. The two surveys involved different intermediate groups - (b) and (c) above - and the main purpose was to assess the effects of single versus double hydraulic retention. They are distinguished mainly for this purpose, rather than the evaluation of possible seasonal fluctuations of the study outcomes. Other variables (i.e. socioeconomic, hygiene and sanitation) were analysed as confounders using a multivariate model. In young children the prevalence rates of A. lumbricoides infection were considerably higher in the raw wastewater group (13.7%), and lower prevalences were observed with decreasing exposure (11.8% in the single reservoir group, 3.3% in the double reservoirs group, and 0.6 - 2.5% in the control group). A high prevalence rate of G.lamblia was observed in children (17 - 20.5%), but no association with untreated wastewater was found. The prevalence rates of E. histolytica infection in children from the various exposure groups ranged between 4.8 - 7.0%, but were considerably higher in older individuals: 15.7 and 16.5% in the two surveys among the raw wastewater group, compared with 13.2% and 14.7% respectively in the controls. In addition, a high prevalence of diarrhoeal diseases (two-weeks recall period) was found in the rainy season, particularly in young children from the raw wastewater exposure group, and lower prevalences were observed with decreasing exposure (29.0% in the raw wastewater group, 26.8% in the two reservoirs group, and 23.0 % in the control group, respectively). The overall prevalences of Cryptosporidium parvum and Trichuris trichiura infections were unexpectedly low (below 1% and 4%, respectively), and excluded from further consideration. The intensity of A. lumbricoides infection was evaluated in a parallel study, and is not reported here. The main findings of the present study can be summarised as follows: - Cropland irrigation with raw wastewater was strongly associated with A. lumbricoides infection in farmworkers and their families, with a risk of diarrhoeal diseases, and with a small but significant risk of E. histolytica infection in individuals aged over 5 years. - The differences observed in the prevalences of A. lumbricoides infection and diarrhoeal diseases were similar in both seasons, but the prevalences in the control group were lower in the dry season; thus, the relative effect of wastewater use was greater in the dry season. - Retention of wastewater in two reservoirs in series (2-6 months) reduced substantially the risk of A. lumbricoides infection, and to a lesser extent the risk of E. histolytica infection, and possibly the risk of diarrhoeal diseases in young children. - Retention of wastewater in a single reservoir (1 - 7 months) did not reduce the risk of A. lumbricoides or E. histolytica infection, but may reduce the ri sk of diarrhoeal diseases in children by 20%. - No association between exposure to raw wastewater and infection with G. lamblia was detected in this research. - Parasitic intestinal infections and diarrhoeal diseases showed significant associations with variables describing personal and domestic hygiene, basic sanitation and socioeconomic characteristics. These results are discussed in relation to local regulations and health protection measures, as well as in light of the WHO 1989 revised guidelines for restricted crop irrigation.