To investigate the chronological changes in conjunctival epithelium after supplementation with a massive oral dose of vitamin A, conjunctival impression cytology (CIC) with transfer was carried out repeatedly among 200 children aged 6-120 months in a randomized, double-blind, placebo-controlled study in Chandigarh (India). Significant conversion to normal CIC started 71-80 d after vitamin A supplementation and by 101-110 d conversion had taken place in all children. Compared with the placebo group, plasma retinol concentration at 100 d post-supplement was found to be significantly higher in the vitamin A-supplemented group P = 0.04. This study demonstrates that CIC responds to a massive oral dose of vitamin A 3-4 months after supplementation. These findings should guide future studies and evaluations in which CIC is used to assess response to vitamin A interventions.