The estimation of placental leucine aminopeptidase (P-LAP) was carried out on pregnancy sera in order to assess its clinical usefulness. The P-LAP level rose progressively with advancing gestation, reaching the highest value (100 unit) at week 39. P-LAP levels in prolonged pregnancies, in which a healthy baby was born, were quite similar to normal pregnancy. On the other hand, the onset of labor in 4 prolonged pregnancies, in which severe neonatal distress occurred, began 14 days or more after the highest value for the daily mean P-LAP levels, and the levels at onset of labor were lower than normal pregnancy. The P-LAP levels in threatened premature delivery were in the order normal pregnancy, those treated only with bed rest and those treated with administration of isoxsuprine. There were no differences in P-LAP levels between normal and mild pre-eclampsia up to week 33, but thereafter the levels for the mild pre-eclampsia were higher than for the normal pregnancy. The P-LAP levels for the severe pre-eclampsia reached their maximum at week 31. Around this week, the levels for severe pre-eclampsia were higher than in the normal pregnancy. After week 35, the levels dropped precipitously to week 40. The levels for severe pre-eclampsia at week 39 and 40 were lower than in normal pregnancy.