As part of an evaluation of a water supply and sanitation project, a baseline cross-sectional study of diarrhoea, and its putative risk factors, was conducted in 5 villages in Imo State, Nigeria. Data were collected from 4641 and 5920 persons during surveys in the dry and wet seasons, respectively. 8 d period prevalence rates for diarrhoea ranged from 5 to 50%, with the highest rates occurring in the 6 to 23 month age group. Diarrhoea was associated with up to 75% of all illnesses in young children and with about 20% in adults. Risk factors included lower socio-economic status, an unclean domestic environment, use of non-purified water, absence of soap, and feeding methods other than exclusive breast-feeding in the early months of infancy. These results suggest that the education component of water supply and sanitation projects should emphasize personal and domestic hygiene and infant feeding.