Reports from various parts of Africa have documented the epidemiological features of leukaemia as including: infrequent diagnosis of acute lymphoblastic leukaemia (ALL) below the age of 5 years; frequent association of acute myelogenous leukaemia (AML) with chloromas; frequent occurrence, predominantly in women, of chronic lymphocytic leukaemia (CLL) below the age of 50 years. The biological and epidemiological features of leukaemia subtypes were determined in 146 patients who were seen prospectively between July 1978 and June 1982. There were 44 cases of chronic myelocytic leukaemia (CML), 34 of ALL, 33 of AML and 31 of CLL. The age distribution and incidence of CML in Ibadan was similar to those of the Black and White populations of the United States. The incidence of ALL in 0-4 year-old Ibadan children was estimated to be less than one-third and one-tenth of those of Black and White children in the United States, respectively, but the incidence of the disease was similar for the 3 populations in the third quinquennium. AML appeared to be more prevalent in 5-9-year-old Nigerian children than in children in the United States and was associated with chloroma in 5 of 9 (55.6%) children in the age-group. As a group, children with ALL were of significantly higher socio-economic status than those with AML. CLL occurred below 50 years predominantly in women (male:female = 1:6) who were significantly of lower socioeconomic status than their CML counterparts. Male patients predominated (male:female = 5.3) at and above 50 years. Numerous factors indicating a poor prognosis co-existed in all ALL patients, including male sex (25/34), WBC greater than 10(10)/litre (31/34, greater than 10(11)/litre (10/34), L2 or L3 morphology (21/25), periodic acid Schiff (PAS) negativity (15/19) and tissue invasion (15/34), thus giving the impression that ALL in young Nigerians is predominantly of an aggressive and hyperproliferative type. The epidemiological features of ALL and CLL in Africans suggest a role for the influence of life-style in leukaemogenesis while the clinical patterns of these disorders suggest that the biological characteristics differ from those of similar diseases in developed countries.