To assess whether Mycoplasma hominis is associated with preterm labour in primigravidae and multigravidae with previous midtrimester abortion or preterm labour.Cohort analytical study.Tygerberg Hospital, a tertiary academic hospital in the Western Cape.Gram's stains were done on smears taken from the posterior vaginal fornix, at the first antenatal visit, between 16 and 26 weeks' gestation, in primigravidae and multigravidae at risk for preterm labour. Cultures for M. hominis and other commonly occurring organisms were done from endocervical swabs taken at the same visit. The outcome of pregnancy in mothers with positive cultures for M. hominis was then compared with outcome in women with negative cultures.Prevalence of Chlamydia trachomatis, Ureaplasma urealyticum, bacterial vaginosis and preterm delivery, birth weight and perinatal deaths.Cultures for M. hominis were positive in 83 patients (21%) and negative in 312 (79%). Significantly more mothers in the positive group (40%) delivered before 37 weeks' gestation than in the negative group (28%, P = 0.0313). Their babies weighed significantly less (2,669 g v. 2,864 g, P = 0.0141). The positive group was also associated with more alcohol use in pregnancy and fewer of them were married. C. trachomatis was found in 18% of mothers in the positive group but in 8% of the negative group (P = 0.0082). U. urealyticum was cultured in 96% of mothers in the positive group in contrast to 81% in the negative group (P = 0.001). Bacterial vaginosis was observed on 75% of mothers with positive cultures for M. hominis but in 22% with negative cultures (P = 0.00001, odds ratio 10.21, 95% confidence interval: 5.63-18.65).Positive culture for M. hominis was associated with more preterm deliveries and also with a higher frequency of C. trachomatis, U. urealyticum and bacterial vaginosis.