Brucella species occasionally cause spontaneous human abortion, but theories regarding whether they do so more frequently than do other infectious pathogens remain controversial. We reviewed 92 pregnant women who presented with acute brucellosis at a Saudi Arabian hospital. From 1983 through 1995, the cumulative incidence of pregnancy and brucellosis was 1.3 cases per 1000 delivered obstetrical discharges. The incidence of spontaneous abortion in the first and second trimesters was 43%, and the incidence of intrauterine fetal death in the third trimester was 2%. Antepartum antimicrobial therapy with cotrimoxazole or cotrimoxazole/rifampin was protective against spontaneous abortion (relative risk, 0.14; 95% confidence interval, 0.06--0.37; P<.0001). The beneficial effect of treatment occurred in women with febrile illness; vaginal bleeding at presentation usually led to spontaneous abortion. This study demonstrated that the incidence of spontaneous abortion among pregnant women with brucellosis is high and that these women should receive prompt therapy with antimicrobial agents when they present for medical care.