The effect of vitamin A supplementation on selected factors of immunity was tested in African children (ages 4 to 24 months with complicated measles) during a randomized double-blind intervention trial. Placebo (n = 31) and treated groups (n = 29) had similar baseline characteristics. The supplemented group had significant reductions in morbidity (expressed as integrated morbidity scores) during the acute (Day 8, P = 0.006) and chronic (Day 42, P = 0.02; 6 months; P = 0.002) phases. In the treated group there was an increase in total number of lymphocytes (Day 42, P = 0.05) and measles IgG antibody concentrations (Day 8, P = 0.02), both of which have consistently been previously shown to correlate more closely with outcome in measles than other immunologic, clinical and radiologic factors. Interleukin 2 and plasma complement values were unaffected by vitamin A supplementation. These findings reinforce results from animal studies that show that the pathways of vitamin A activity in decreasing morbidity and mortality are partly founded on selective immunopotentiation.