There is controversy concerning the role of sexual intercourse (SI) in mid- pregnancy and spontaneous labor onset (LO). Sexual orgasm-related release of prostaglandins, male-superior position (MSP) during SI, and infections have been implicated. The objectives of this study were to test: 1) the risk for LO from SI; and 2) whether cocaine abuse increases such risk. Study Design: We prospectively interviewed 100 women within 24 hrs of spontaneous LO at 22-35 weeks of pregnancy; 312 women not in labor, matched for ethnicity, age, and pregnancy duration, attending a prenatal clinic were controls. A battery of in-depth and directed questions related to SI and life-style habits was used. Target period was up to 7 days before the event day (defined as LO-day in cases and the interview-day in controls). Frequencies of SI, condom use, orgasm, MSP, and cocaine abuse were compared within groups (1st vs 7th day before the event day, a case-crossover design) and between groups (case-controlled design). Results: The distribution of age, gestation, and race were similar. 5--10% in each group reported SI for each of the 7 days. For the 7-day period, one, two, and three or more times SI activity was reported by 26%, 11%, and 2% of cases, and 26%, 18%, and 13% of controls, respectively (p=ns). Within 24 hrs prior to the event 9% of cases and 7% controls reported having SI; 7th day prior to the event 6% cases and 15% controls reported having SI (p=ns). No increased risk of LO was observed following SI, either one day, or at any time during the week preceding the event, based on logistic regression analysis with and without controle for race and age. 61% of cases and 42% of controls reported no SI in the entire week preceding the event (p=ns). Condom use was 0--10% in cases and 2-12% in controls (p=ns). Frequenies of MSP during SI (0--70%), and orgasm (25--75%) were similar between groups. No controls and 2% of cases reported cocaine abuse 1 day prior LO---none reported SI with it. Overall cocaine abuse frequency (history) was only 1--4%. Conclusion: 1) In this study we found no clear association between SI and spontaneous LO; and 2) this finding has important epidemiological implications.