Background: To include the patient perspective in the assessment of adverse events in oncology, a patient-reported outcomes (PRO) version of the Common Terminology Criteria for Adverse Events (CTCAE) was developed by the US National Cancer Institute, the so called PRO-CTCAE. The objective of this study was the development of disease-specific PRO-CTCAE item sets for patients with breast cancer (BC), multiple myeloma (MM), and prostate cancer (PC). Methods: The cross-sectional survey was conducted at three German outpatient cancer centers. Prevalence and importance of the 78 PRO-CTCAE symptoms were assessed using a patient questionnaire. To select the most relevant PRO-CTCAE items for each tumor entity, symptoms were ranked based on patient answers. Results: 101 patients with BC, 107 with MM, and 66 with PC participated. The final item sets contained 21 symptoms (BC) or 19 symptoms (MM and PC), respectively. Eight symptoms (fatigue, muscle pain, insomnia, joint pain, general pain, dizziness, shortness of breath, and swelling) were represented in all three item sets. Fatigue was the symptom with the highest ranking across item sets followed by insomnia. Symptoms with the highest rankings represented in only one item set were symptoms affecting the urogenital system in the PC item set, blurred vision in the BC item set, and decreased appetite in the MM item set. Conclusions: Individual PRO-CTCAE item sets for a German patient population were developed for the three tumor entities on the basis of patients’ differences in symptom profiles and perceptions. The quality and psychometric criteria of the newly compiled item sets should be evaluated in validation studies.