Studies of Down's syndrome covering a period of 9 years revealed an incidence of 1 in 865 livebirths in a Nigerian hospital. Cytogenetic analysis in 386 patients showed 369 (95.5%) cases to be the result of regular trisomy 21, and translocation trisomy 21 was found in nine (2.5%) patients. Six (1.5%) patients were mosaics and the remaining two (0.5%) cases were classified as miscellaneous. A high incidence of cases among young mothers was recorded, but a search for environmental factors contributory to non-disjunction in this relatively young age group was unrevealing. The study has shown that Down's syndrome occurs as commonly in Negroes as in other races. Epidemiological studies like this are necessary to heighten the awareness of health planners in communities that have for a long time considered haemoglobinopathies to be the major genetic disorder, in order to prepare the ground for preventive measures.