The aim of this study was to investigate if previously oligo- or amenorrhoeic polycystic ovary syndrome (PCOS) patients gain regular menstrual cycles when ageing. Women registered as having PCOS, based on the combination of oligo- or amenorrhoea and an increased LH concentration, were invited by letter to participate in a questionnaire by telephone. In this questionnaire we asked for the prevalent menstrual cycle pattern, which we scored in regular cycles (persistently shorter than 6 weeks) or irregular cycles (longer than 6 weeks). We interviewed 346 patients of 30 years and older, and excluded 141 from analysis mainly because of the use of oral contraceptives. The remaining 205 patients showed a highly significant linear trend (P < 0.001) for a shorter menstrual cycle length with increasing age. Logistic regression analysis for body mass index, weight loss, hirsutism, previous treatment with clomiphene citrate or gonadotrophins, previous pregnancy, ethnic origin and smoking showed no influence on the effect of age on the regularity of the menstrual cycle. We conclude that the development of a new balance in the polycystic ovary, solely caused by follicle loss through the process of ovarian ageing, can explain the occurrence of regular cycles in older patients with PCOS.