OBJECTIVE: To compare the effects of forceps delivery and spontaneous delivery on pelvic floor functions in nulliparous women. DESIGN: A longitudinal prospective study with investigations during the first pregnancy, 10 weeks and 10 months after delivery. SETTING: Antenatal clinic in a teaching hospital. POPULATION: One hundred and seven patients aged 28 +/- 4 years, divided into those with forceps (n = 25) or spontaneous (n = 82) delivery. METHODS: Investigations with a questionnaire, clinical examination, assessment of bladder neck behaviour, urethral sphincter function, intra-vaginal/intra-anal pressures during pelvic floor contractions. RESULTS: The incidence of stress urinary incontinence was similar in both groups at 9 weeks (32% vs 21%, P = 0.3) and 10 months (20% vs 15%, P = 0.6) after delivery, as was the incidence of faecal incontinence (9 weeks: 8% vs 4%, P = 0.9; 10 months: 4% vs 5%, P = 1) and the decreased sexual response at 10 months (12% vs 18%, P = 0.6). Bladder neck behaviour, urethral sphincter function and intra-vaginal and intra-anal pressures were also similar in the two groups. However, 10 months after delivery, the incidence of a weak pelvic floor (20% vs 6%, P = 0.05) and the decrease in intra-anal pressure between the pre- and post-delivery values (-17 +/- 28 cm H2O vs 3 +/- 31 cm H2O, P = 0.04) were significantly greater in the forceps-delivered women. CONCLUSIONS: Forceps delivery is not responsible for a higher incidence of pelvic floor complaints or greater changes in bladder neck behaviour or urethral sphincter functions. However, patients with forceps delivery have a significantly greater decrease in intra-anal pressure and a greater incidence of a weak pelvic floor.