BACKGROUND: Malaria infection increases low birthweight especially in primigravidae. Malaria epidemics occur when weather conditions favour this vector borne disease. Forecasting using the El Niño Southern Oscillation (ENSO) may assist in anticipating epidemics and reducing the impact of a disease which is an important cause of low birthweight. The aim of the present study was to determine the impact of the malaria epidemic in East Africa during 1997-1998 on birthweights in two different areas of Tanzania and to explore ESNO's potential for forecasting low birthweight risk in pregnant women. METHOD: A retrospective analysis of birthweight differences between primigravidae and multigravidae in relation to malaria cases and rainfall for two different areas of Tanzania: Kagera, which experiences severe outbreaks of malaria, and Morogoro which is holoendemic. Birthweight and parity data and malaria admissions were collected over a 10-year period from two district hospitals in these locations. RESULTS: The risk of delivering a low birthweight baby in the first pregnancy increases approximately 5 months following a malaria epidemic. An epidemic of marked reduced birthweight in primigravidae compared with multigravidae occurred, related to the ENSO of 1997-1998. In Kagera this birthweight difference and the risk of low birthweight were significantly lower compared with Morogoro, except after the ENSO when the two areas had similar differences. No significant interaction was noted between secundigravidae and any of the risk periods. The results indicate that the pressure of malaria is much greater on pregnant women, especially primigravidae, living in the Morogoro location. CONCLUSIONS: Surveillance of birthweight differences between primigravidae and multigravidae is a useful indicator of malaria exposure.