Serial monitoring by plasma progesterone measurement is advised in the literature for fertility work-up, to detect ovulation disturbances in women presenting with regular menstrual cycles. Three strategies to diagnose such 'subtle ovulation disorders' (SOD, defined as anovulation, inadequately timed ovulation or ovulation of a follicle of reduced size in regularly cycling women) were evaluated, in order to investigate costs of such a diagnosis. On the basis of a 'maximal', an 'ultrasound-only', and a 'preselection' strategy, total medical costs and costs including non-medical costs were calculated for each SOD diagnosis. A 'maximal' diagnostic strategy resulted in a total medical cost of ECU 9057 per diagnosis (including non-medical costs ECU 12,787); an 'ultrasound-only' strategy in ECU 4520 (ECU 6791) per diagnosis. By use of a 'preselection' strategy, 4.25% of the women were found to have an SOD, at a cost of ECU 3036 (ECU 6868) for each diagnosis. As the real significance of SOD diagnosis for the prognosis of the patient to become pregnant without treatment remains unclear, and as no randomized trials on treatment effectiveness have as yet been undertaken, it is questionable whether this approach is worthwhile.