Plasma and breastmilk (BM) viral titers were obtained from women a median of 8 weeks after single-dose nevirapine. Samples from women with laboratory-diagnosed mastitis had higher viral titers in BM than those without mastitis. In the 20  women with detectable virus in BM, 65% of BM and 50% of plasma RT sequences had NNRTI-resistance mutations, with divergent mutation patterns. The Journal of Infectious Diseases 2005;192:1260-1264