Alterations in the hormonal milieu associated with the menstrual cycle appear to influence the dynamic interaction between the bladder and urethra as well as detrusor function, probably because of the common embryological origin of the lower genital and urinary tracts. In this retrospective study we investigated the effect of the menstrual cycle on cystometric diagnosis. A retrospective case note review of 687 consecutive patients attending the urogynecology unit of St Georges University Hospital, a tertiary referral center, was carried out. The study group comprised 57 women with regular menstrual periods. In both patients whose symptoms were adversely affected premenstrually and those whose symptoms were not influenced by the menstrual cycle, the majority of normal cystometric diagnoses were made in the luteal phase: 45.5% vs. 25% (P < or = 0.002) and 38.5% vs. 4.8% (P < or = 0.05), respectively. Diagnoses of genuine stress incontinence, detrusor instability and mixed genuine stress incontinence and detrusor instability were most frequently made in the follicular phase of the cycle. More normal cystometric diagnoses were made in the influenced group (36.8%) than in the uninfluenced group (21%) (P < 0.02). The results of this preliminary study indicate that the timing of cystometric evaluation may influence the detection of a positive diagnosis. The luteal phase may not be the correct time to make an accurate diagnosis, especially in patients whose symptoms are influenced by their menstrual cycle.