A double blind controlled trial of supplementation with folic acid has been performed in 117 children with homozygous sickle cell (SS) disease aged 6 months to 4 years over a 1 year period. No megaloblastic change was observed in either group. At the end of the study period the folate supplemented group showed no significant differences in haemoglobin, growth characteristics, or in the proportion of children affected by major or minor infections, acute splenic sequestration, dactylitis or episodes of bone or abdominal pain. However, the folate supplemented group showed a significantly lower mean cell volume and the placebo group contained a significant excess of children experiencing multiple episodes of dactylitis. The results are compatible with mild folate deficiency in some patients in the placebo group but the absence of striking effects on haematology or growth suggest that the policy of regular folate supplementation in children with SS disease should be critically reviewed.