Objective. Medical abortion using mifepristone followed by misoprostol is increasingly used for termination of an unwanted pregnancy. Consequently, an increasing number of women undergo medical abortion while still breastfeeding from a previous pregnancy. But there are no data on mifepristone use during lactation. We studied the levels of mifepristone in breast milk collected from women undergoing medical abortion. Design and samples. Samples of milk were collected from 12 women during the first 7 days after intake of either 200 mg (n = 2) or 600 mg (n = 10) of mifepristone. In addition, serum samples were collected on day 3 (n = 4). Main outcome measures. The levels of mifepristone, quantified using radioimmunoassay. Results. The milk concentrations of mifepristone were highest in the first samples collected during the first 12 hours following drug intake, and ranged from undetectable (< 0.013 μmol/l) to 0.913 μmol/l. Thereafter, declining concentrations of mifepristone were detected up to 7 days. The lowest levels of mifepristone in milk were measured following ingestion of the 200 mg dose. The milk:serum ratio of mifepristone ranged from < 0.013:1 to 0.042:1 on day 3 (n = 4). The calculated relative infant dose (RID) was 1.5% at its highest. Conclusions. The levels of mifepristone in milk are low, especially when using the 200 mg dose. Breastfeeding can be safely continued in an uninterrupted manner during medical abortion of this kind.