During a nationwide acetaminophen overdose study conducted at the Rocky Mountain Poison and Drug Center from 1976-1985, 113 patients entered into the study were reported to be pregnant at the time of the overdose. Follow-up, including appropriate laboratory and pregnancy outcome data, was available in 60 cases. Of these, 19 women overdosed during the first trimester, 22 during the second trimester, and 19 during the third trimester of pregnancy. Of the 24 patients with acetaminophen levels above the acetaminophen overdose nomogram line, ten were treated with N-acetylcysteine within 10 hours post-ingestion; eight delivered normal infants and two had elective abortions. Of ten patients treated with N-acetylcysteine 10-16 hours post-ingestion, five delivered viable infants, two had elective abortions, and three had spontaneous abortions. Of four women treated with N-acetylcysteine 16-24 hours post-ingestion, one mother died, and there was one spontaneous abortion, one stillbirth, one elective abortion, and one delivery. Multiple logistic regression demonstrated a statistically significant correlation between the time to loading dose of N-acetylcysteine and pregnancy outcome, with an increase in the incidence of spontaneous abortion or fetal death when treatment was begun late. We recommend that pregnant women who take an acetaminophen overdose and have a potentially toxic serum level be treated with N-acetylcysteine as early as possible.