OBJECTIVE--Provide evidence from prospective data that Neisseria gonorrhoeae may be an important cause of premature delivery and low birth weight in areas with high prevalence of genital infections. SETTING--Department of Obstetrics and Gynaecology, Kalafong University Hospital, Pretoria, South Africa in collaboration with the Departments of Microbiology and of Gynaecology and Obstetrics, Katholieke Universiteit, Leuven, Belgium. SUBJECTS--Two hundred and fifty six consecutive black pregnant women were examined during the first antenatal visit, and one to four weeks later a second culture for N gonorrhoeae was taken at random in 67 of them. Hundred and sixty seven were analysable, 75 were lost to follow up. METHODS--After obtaining detailed clinical history, an endocervical specimen for N gonorrhoeae culture (Thayer-Martin) and C trachomatis antigen detection (Chlamydiazyme (R)) was taken. Syphilis was diagnosed when both reactive plasma protein (RPR) and T pallidum haemagglutination inhibition assay (TPHA) were positive. Prematurity was defined as delivery at less than 37 gestational weeks. RESULTS--Infection with N gonorrhoeae (n = 9) and untreated syphilis (n = 7) were both associated with prematurity and low birth weight. After multi-variate regression analysis, age-adjusted parity, late sexual debut, number of recent sexual partners, infection with N gonorrhoeae and infection with syphilis revealed significant associations with low birth weight. However, infection with C trachomatis, presence of abundant vaginal discharge, social class, Trichomonas vaginalis infection, gestational weeks at first antenatal visit and number of previous miscarriages did not reveal such an association. Attributable risk of untreated gonorrhoea for premature birth was 72% and routine cultures were cost-benefit efficient. CONCLUSIONS--At least in countries where the prevalence is high, genital infections as well as the risk factors for acquiring them (young age, late sexual debut, number of recent partners) play a major role in the aetiogenesis of prematurity and low birth weight. N. gonorrhoeae is a main contributor, and in high prevalence areas it should be routinely looked for and treated for during pregnancy.