Background: Adequate and timely monitoring of adverse events to cancer treatment is from our view dependent on a suitable Patient Reported Outcome (PRO) tool developed for the specific patient population based on cytostatic drugs included in the treatment. Therefore, a systematic method for construction of PRO questionnaires including selection of the appropriate questions is needed. Purpose: The purpose of the present study was to develop and test a method of item selection for a PRO questionnaire to monitor adverse events in oncologic routine treatment of metastatic prostate cancer patients. Patient and methods: Documentation on common symptomatic adverse events for the three drugs was collected from five different sources: 1) FDA product summary information; 2) EMA product summary information; 3) phase 3 Randomized Controlled Trials (RCT) leading to drug approval; 4) audit of the electronic patient files focusing on the oncologist's documentation of adverse events and 5) individual patient interview (n = 16) focusing on adverse events. The Patient Reported Outcome of Common Terminology Criteria for Adverse Events (PRO-CTCAE) was used as PRO item library. Selected symptoms were transformed into corresponding PRO-CTCAE questions. The questionnaire was tested by patients in a pilot test (n = 12). Patients for interviews and pilot testing were included by purposive sampling. Results: A method for constructing a PRO questionnaire was developed, and a questionnaire of 25 PRO-CTCAE symptoms with 46 questions including an open write-in space for additional adverse events was constructed and tested. Conclusion: This study demonstrates a systematic method to select questions on adverse events for a PRO questionnaire in a specific cancer population receiving oncologic treatment. The present study reveals that audit of patient files and patient interviews in our setting only add little to the information on adverse events obtained from FDA, EMA and RCT. The obtained questionnaire was found useful and acceptable by patients.