Recently, the efficacy of oral vitamin A supplementation for measles and respiratory syncytial (RSV) infection has been evaluated in developing countries. However, in developed countries where vitamin A deficiency is little worth consideration, few studies have been conducted on the effect of vitamin A supplementation. The effect of oral vitamin A (100,000 IU) supplementation was evaluated in 105 children with measles (age 5 months to 4 years) and in 96 children with RSV infection (ages a month to 2.5 years) in Fukushima, Japan. Comparisons were made of clinical signs, duration of hospitalization and complications between treated groups and non-treated groups. Treated group (measles n = 47, RSV n = 54) and non-treated groups (measles n = 58, RSV n = 42) had similar baseline characteristics. Patients with measles given a vitamin A supplementation had a shorter duration of cough (7.2 +/- 1.6 vs 9.2 +/- 1.8 days, p < 0.05) and patients with severe RSV infection given a vitamin A supplementation had a shorter duration of retraction (3.6 +/- 1.4 vs 5.3 +/- 0.8 days, p < 0.05) and wheezing (4.4 +/- 1.7 vs 6.3 +/- 1.5 days, p < 0.05). Toxicities, including excess vomiting and bulging fontanel were not observed. Our findings may suggest the efficacy of oral vitamin A supplementation for measles and severe RSV infection, in children who have no malnutrition.