OBJECTIVES: To compare the results obtained with the IPSS administered by GP (GP-IPSS) versus a self administered questionnaire (patient-IPSS). METHODS: A total of 338 recently diagnosed BPH patients were involved in the analyses. For each of them, their GP (General Practionner) filled an GP-IPSS questionnaire in according to their knowledge of their patients. The patient completed the patient-IPSS questionnaire himself and send it back to data management center through a prepaid envelope to avoid patient's answers to be viewed by the GP. Bland & Altman method was chosen to visualize individual difference and to quantify the bias. RESULTS: On average GP overestimated by 1.3 points [0.9-1.7]95% the IPSS compared to patient self evaluation. Individual discrepancies reached 3 points or more among 42% of patients. The results of GP-IPSS were significantly higher at 6 months, 12 months and 18 months taking into account Bonferoni correction. CONCLUSIONS: There was a statistically significant bias between GP's administered IPSS and self administered IPSS. We recommend to always keep the same IPSS administration's mode during the course of a clinical trial.