Newer antipsychotic drugs offer significant clinical advantages for the treatment of psychosis. In particular for the treatment of postpartum disorders newer agents may be suited due to their favourable side-effect profiles. Of concern is the passage of the drugs into breast milk and what potential risks this poses for an infant who is breastfed. The excretion of olanzapine into the breast milk of five lactating women with postpartum psychosis was examined in this study. Nine pairs of plasma and breast-milk samples were collected and the concentration of olanzapine determined by high-performance liquid chromatography. Single-point milk-to-plasma ratios were calculated and ranged from 0.2 to 0.84 with a mean of 0.46. The median relative infant dose was 1.6% (range 0-2.5%) of the weight-adjusted maternal dose. During the study period, there were no apparent ill effects on the infant as a consequence of exposure to these doses of olanzapine. As with other antipsychotic drugs this study demonstrates that olanzapine passes into breast milk. The long-term effects of exposure in infants exposed to olanzapine requires further investigation.