OBJECTIVES: In a real world setting, to design a system to measure the effect of fluctuating symptoms (problem periods) by capturing information on days of the largest and smallest impact. METHODS: The evaluation was conducted throughout the United Kingdom using a web-based system – PROBE (patient reported outcome based evaluation). We investigated women reporting problems with their periods. The issues in setting out and programming the study design were  a) determining the day on which they would suffer the greatest impact, b) duration of impact c) predicting when the normal data could be collected. Variability exists between woman to woman and between cycles for each woman. The problems were addressed by an initial screening questionnaire, which determined time between menses, length of menses, regularity of menstrual cycle and date of next menses so that questionnaires could be sent out on the appropriate days.  5 consecutive days of questionnaires were completed during menses and 3 consecutive days between menses. Measures included demographic data, symptoms, previous treatments, current medications, absenteeism, presenteeism, HRQoL using SF 36 and EQ 5D. RESULTS: Target numbers were 2000 for the screening questionnaire and 200 for the main survey, in reality 2699 women participated in the screening questionnaire with 331 in the main survey.  165 women completed the survey at both the time of menses and between menses allowing a direct comparison of their reported outcomes. On the validated questionnaires the data quality was high as reported by the Copyright holders. Significant findings were obtained on the primary and secondary measures. CONCLUSIONS: This evaluation shows that a web based survey system (PROBE methodology) is flexible, interactive and reliable in obtaining patient reported outcomes in a condition with fluctuating symptoms.