PIP: In this prospective study, an attempt was made to determine the relationship between coitus and onset of labor. 140 pregnant women at and beyond 28 weeks of pregnancy were studied. Women with clinically evident causes of preterm labor like incompetent os, hydramnios, twins, and antepartum hemorrhage were excluded. Cases with abnormal presentations were also excluded. These patients knew their last menstrual period. Detailed history of any symptoms of lower genital tract infection was requested. 79.29% of the women were between 20 and 29 years of age. Both primigravidas and multigravidas were included. Out of 42 women with history of regular coitus during pregnancy, 17 women (40.48%) had preterm onset of labor, while 30 out of 98 (30.61%) had preterm onset of labor. There was no difference in the behavior of primigravidas and multigravidas. However, it was found that women who had coitus and symptoms of lower genital tract infection had a significantly higher incidence of preterm labor (46.88%) as compared with women with sexual activity but without such symptoms (20.O%) (p 0.005). When symptomatic women with positive sexual activity and negative coital history were compared, the incidence was similar: 46.88% and 48.07%, respectively. However, when symptomatic women with a positive history of sexual activity were compared with asymptomatic women without sexual activity, the occurrence of preterm labor was significantly more in the positive group: 20% vs. 10.87% in the negative history group (p .05). The significantly higher increase in preterm labor in women who had symptoms of lower genital tract infection and coitus implies that semen rather than coitus may affect the onset of labor. Further studies are required to learn more about this observation.