The effect of coitus on precipitation of preterm labor was studied prospectively in 126 women with twin pregnancies. All of the women attended a special antenatal clinic in a tertiary referral center and were questioned about the frequency of coitus. Of the participants, 40% reported coitus early in the third trimester; the rate decreased to 24% by 36 weeks' gestation. There were no significant differences in the frequency of positive responses between those who went into labor before term when contrasted with those who were delivered of infants at term. The data indicate that coitus is not an important precipitant of preterm labor in this high-risk group and that coitus need not be discouraged in women with twin pregnancies.