The aim of the study was to identify the possible relationship between body mass index and intra-abdominal pressure as measured by multichannel cystometry. A retrospective chart review of patients presenting for urodynamic evaluation between January 1995 and March 1996 was carried out. Variables identified included weight, height, intra-abdominal pressure and intravesical pressure as recorded on multi-channel cystometrogram at first sensation in the absence of detrusor activity. Body mass index was defined as weight in kilograms divided by height in square meters. Intra-abdominal pressure was measured intravaginally except in those cases of complete procidentia or severe prolapse, where it was measured transrectally. Adequate data were available on 136 patients. The mean age was 60.6 years (range 30-91); mean body mass index was 27.7 kg/m2 (range 12.7-47.7); and mean intra-abdominal pressure was 27.5 cmH2O (range 9.0-48.0). A strong association between intra-abdominal pressure and body mass index was demonstrated, with a Pearson coefficient correlation value of 0.76 (P<0.0001). Strong correlation was still demonstrated when those patients who had had the intra-abdominal pressure measured transrectally were separated out, thus eliminating any possible confounding factors between measurements of intra-abdominal pressure measured transvaginally versus transrectally. In addition a strong correlation between intravesical pressure and body mass index was also demonstrated, with a Pearson coefficient correlation value of 0.71 (P<0.0001). Of the 136 patients, 65 (47.8%) were ultimately diagnosed as having genuine stress urinary incontinence (GSUI), 35 (25.7%) with GSUI and a low-pressure urethra (maximum urethral closure pressure of less than 20 cmH2O), and 18 (13.2%) with detrusor instability. The remaining 13.2% had severe prolapse. Our data demonstrate a significant correlation between body mass index and intra-abdominal pressure. These findings suggest that obesity may stress the pelvic floor secondary to chronic state of increased pressure, and may represent a mechanism which supports the widely held belief that obesity is a common factor in the development and recurrence of GSUI.