Method: RO treated in our centres were reviewed, and we focused on identifying the worrisome and the atypical features. Results: We identified 9 RO with at least one of the worrisome feature. Patients: 5 M/4 F (mean age 72,89 year; range: 63-88). Mean tumor size: 4,24 cm (range: 1,6-11,5 cm); right kidney: 5, and left: 4 cases. Invasion into the perinephric or renal sinus fat was the most frequent worrisome feature: 7 tumors (77,8 %) and focal chromophobe carcinoma-like areas in 3 cases (33,3 %). Lymphovascular invasion, entrapped renal tubules-glomeruli, necrosis and mitosis were found in 2 (22,2 %) respectively. All but two ROs had at least 2 worrisome features. Followup was available for all cases (median 11 months; range 3-108): all patients were alive without recurrence or metastasis. Conclusion: RO is a tumor which often can show worrisome and atypical morphology. It is necessary to know and recognize the worrisome features to prevent diagnostic errors, if otherwise typical oncocytoma morphology is present. Despite these atypical morphological data, the prognosis is excellent.