Many patients with sickle cell anaemia (SCA) are known to synthesize increased amounts of foetal haemoglobin (Hb F). In some situations, the levels attained are so high that the course of the disease is ameliorated since Hb F does not participate in the polymerization process characteristic of the sickling phenomenon. It has also been reported that the simultaneous inheritance of an alpha-thalassaemia gene reduces the severity of SCA. We have examined the levels of Hb F in relation to the erythrocyte indices and the coinheritance of the deletion type alpha-thalassaemia in SCA patients in Nigeria. The concentration of Hb F in peripheral blood was measured by the alkali denaturation technique of Betke et al. [15], whilst erythrocyte indices were determined on a Coulter S plus II counter. Alpha-thalassaemia was detected by the restriction endonuclease analysis of DNA obtained from peripheral white blood cells (WBC) and nucleated red cells using alpha-globin gene-specific probes. The mean Hb F level in 130 SCA subjects was 5.9 +/- 3.8% (range 0.9-16%). Males had significantly lower levels than females. Hb concentration, haematocrit, and Hb A2 did not differ in subjects with Hb F levels lower than 2% (Group I) when compared with those whose Hb F levels were higher than 8% (Group II). The mean corpuscular volume (MCV) and mean corpuscular haemoglobin (MCH) were lower in Group I. Globin analysis in 30 of these subjects showed that 20 had four, eight had three, and two had two alpha-globin genes.(ABSTRACT TRUNCATED AT 250 WORDS)