Alteration in the hormone level associated with menstrual cycle influences the interaction between the urethra and bladder as well as detrusor function, maybe due to the common embryological origin of lower female genital and urinary tract. We tried to investigate the effect of the menstrual cycle on cystometric diagnosis through this retrospective study. 60 women with regular menstruation were enrolled in this study. The study groups were divided into 2 groups, Group I was women whose symptoms were not influenced by the menstrual cycle, Group II was women whose symptoms were adversely affected premenstrually. The majority of normal cystometric diagnosis were made in the luteal phase (Group I: 42.9% vs 4.4%, p<0.05; Group II: 50.0% vs 22.2%, p<0.05). But diagnosis of genuine stress incontinence, detrusor instability, mixed genuine stress incontinence and detrusor instability were frequently made in the follicular phase of mentruation. Normal cystometric diagnosis in the group II were more commom than the group II ( 36.8% vs 19.7%, p<0.05 ). The results of this study reveal that the timing of cystometric evaluation may influence the the detection of a positive diagnosis. In patients whose symptom are influenced by their menstrual cycle, the luteal phase may not be the correct time to make an accurate diagnosis.