One hundred fourteen Ethiopian women with leprosy and 33 healthy women without leprosy were studied prospectively throughout 119 and 37 pregnancies, respectively, and followed up during lactation. Fifty-five women showed worsening of their leprosy status; in 31 (56%) this was observed during the third trimester of pregnancy. Forty women were diagnosed as having Type 1 lepra reaction; in 20 (50%) the first occurrence was during the first six months of lactation. Twenty-eight women had Type 2 lepra reaction, which in 19 (68%) first occurred during the third trimester of pregnancy or the first six months of lactation. These adverse effects of pregnancy on leprosy are thought to be associated with suppression of maternal cell-mediated immunity during gestation and recovery postpartum. Implications for the obstetrician, physician and leprosy health worker are discussed.