A cross-sectional study, a follow-up study, and an intervention trial were carried out to investigate the association between mild vitamin A deficiency and the occurrence of diarrhea and respiratory diseases. Cross-sectional analysis was performed among 1,772 children, aged 1–8 years, in the Sakon Nakhon province of northeastern Thailand. Children with a history of diarrhea or respiratory disease had lower levels of serum retinol and retinol-binding protein. Adjusted for age, sex, nutritional status, and level of urbanization, logistic regression using data for 877 children showed a negative association between serum retinol and both diarrhea and respiratory diseases. A follow-up three months later (n = 146 children) showed that children with deficient serum retinol (<0.35 μmol/liter) had a fourfold greater risk of respiratory disease (p < 0.01). No relation was found for diarrhea. An intervention trial (n = 166 children aged 1–5 years) showed that, during 2 months of follow-up after administration of oral vitamin A (200,000 IU), the control group (aged 3–5 years) had a higher incidence of respiratory disease (2.9 times) as well as diarrhea (3.1 times). Between 2 and 4 months, a significantly (p < 0.025) higher incidence of respiratory diseases (2.5 times) could be observed in children aged 1–2 years. This study supports earlier reports on a greater risk of respiratory diseases and of diarrhea in mild vitamin A deficiency. Supplementation reduced the incidence of both diarrhea and respiratory disease for a period of at least 2 months.