Observational studies have suggested that the transmission of the human immunodeficiency virus (HIV) is enhanced in the presence of other sexually transmitted diseases (STDs). Based on this STD/HTV co-factor hypothesis it has been suggested that interventions aimed at reducing the prevalence of STDs may be able to reduce the incidence of HTV infection. This thesis presents the results of a community based randomised controlled trial of the impact of improved STD case management on the incidence of HTV infection. The trial was earned out in the Mwanza Region of North-Western Tanzania between 1991 and 1995. It involved 12 rural communities which were formed into six matched pairs. Within each pair, one community was randomised to receive the intervention which comprised improved STD case management services integrated into the existing primary health care (PHC) structure, and health education campaigns to improve treatment seeking behaviour for STDs. The impact of the intervention was studied in a cohort of about 1000 adults aged 15 to 54 years from each of the communities. Over the 2 years of follow-up, there were 130 HIV seroconversions, 48 (1.16%) in the intervention group and 82 (1.86%) in the comparison group, equivalent to annual incidences of 0.58% and 0.93%. The crude relative risk (RR) for seroconversion in intervention compared to comparison communities was 0.57 (95% Cl 0.42- 0.76, p=0.004) After adjustment for potential confounders, the RR was 0.62 (95% Cl 0.45- 0 85, p=0.013), equivalent to a reduction of 38%.There was also a reduction in the prevalence of active syphilis (adjusted RR = 0.67; 95% Cl: 0 47-0.96, p=0.04 at RPR titre of >=1:8), and of symptomatic urethritis in men (adjusted RR = 0.51, 95%C1: 0.25-1.03,p=0.06). No impact was observed for gonococcal / chlamydial infection or overall urethritis in men. There was no impact on the prevalence of STDs in pregnant women, studied by means of two consecutive cross-sectional studies at antenatal clinics. The results of this trial provide additional strong evidence for the STD/H1V co-factor hypothesis, and demonstrate that improving STD services can substantially reduce the incidence of HTV infection in populations with high STD prevalences. They suggest that the control of STDs should form an important component of HIV prevention programmes Reliable drug supply and frequent support supervision to health facilities are essential prerequisites for such STD control programmes.