Rapid urbanization has created numerous health risks in developing countries, but the exact impact on many diseases in function of living conditions is unclear. For insight into this complex relationship, a study on diarrheal diseases was carried out to obtain knowledge about the distribution of health risks in an urban setting. An epidemiological survey with a combined longitudinal and transverse design was conducted in Rufisque, Senegal, from April 2002 to March 2003 in a sample including households with children less than 5-years-old living in four areas presenting different levels of hygiene. Results showed a high overall incidence of diarrhea (6.5 episodes/child/year) but there were major discrepancies between the four study areas in direct relation with level of hygiene. The annual incidence per child was lower in the low-cost housing project (fair hygiene, 3.4 episodes) than in the Castors area (poor hygiene, 6.8 episodes), Diokoul Wague area (very poor hygiene, 7.3 episodes) and Goufe Aldiana area (no hygiene, 8.4 episodes). The study showed only a slight seasonal effect on diarrheal disease in the different areas. However, the differences observed between areas during the cold and hot dry seasons were considerably attenuated in the rainy season. This variability in the incidence rate that underlines the diversity of urban living conditions depends on a variety of risk factors (such as age and number of children) that may interact, although hygiene level remains critical. For issues usually given priority at the national level, multiplying studies aimed at fine analysis of factors underlying disease transmission is useful since this approach can improve understanding of public health policy in city environments characterized by the complex conditions (density and diversity) created by urbanization.