Vitamin A deficiency can occur during lactation and breast-milk vitamin A has been recommended for monitoring the vitamin A status of lactating women and their infants. This study aimed to investigate the vitamin A status of lactating women in relation to race, age, parity, duration of lactation and anthropometric status. A cross-sectional study was conducted among 262 lactating women in rural Chiang Mai, Thailand. Blood and breast-milk samples were collected. Serum retinol, carotene and breast-milk retinol concentrations were analysed by high-performance liquid chromatography. The results show that mean serum retinol and breast-milk retinol in hill tribes were significantly lower than in Thais, 1.91 (0.59) and 0.79 (0.52) compared with 2.10 (0.51) and 1.04 (0.58) μmol/L, respectively. Mean serum retinol and breast-milk retinol were highest during the 1st 3 months of lactation. Maternal age, parity and anthropometric status (BMI) were not associated with serum or breast-milk retinol concentrations. There was a significant relationship between serum and breast-milk retinol values in women who breastfed for 6 months or longer (regression co-efficients 0.30; 95% CI 0.16, 0.43). Breast-milk retinol levels declined significantly from 4 to 12 months after delivery, which could increase the risk of vitamin A deficiency in children who were exclusively breastfed or receiving inappropriate complementary foods during this period. Weaning foods which commence at 6 months and have an adequate vitamin A content should ensure that the vitamin A status of the young child is maintained.