© The Foundation Acta Radiologica 2020.Background: According to sick lobe theory, one or more lobes of the breast are more prone to the development of carcinoma. However, the implications of this theory in breast magnetic resonance imaging (MRI) are unknown. Purpose: To evaluate the MRI appearance of mass type (multifocal and multicentric diseases) and non-mass type (non-mass enhancements) sick lobe patterns, together with the histopathology results. Material and Methods: MRI reports of 2015 patients in two tertiary breast imaging centers between June 2012 and June 2018 were retrospectively reviewed for multifocal–multicentric diseases and segmental, linear, and regional enhancements. A total of 113 patients were included. The specimens obtained by thick needle, vacuum, excisional biopsy/lumpectomy or mastectomy after breast MRI scans were pathologically assessed. The pathologic results were categorized as invasive carcinoma, precursor, and benign proliferative lesions according to the 2012 World Health Organization Classification of Tumors. Results: The percentage of underlying benign and precursor invasive lesions was significantly different in patients with mass and non-mass MRI patterns. While the pathology results of mass type patterns were premalignant and malignant in all cases, nearly half of the underlying histologies were benign proliferative subtypes in patients with non-mass type patterns. Conclusion: In this study, the mass and non-mass patterns derived from sick lobe theory were related to different risks of malignancy in the pathological examinations.