OBJECTIVE To identify histopathologic correlates for the varied magnetic resonance (MR) imaging appearances of fibroadenomas. METHODS Twenty-three fibroadenomas in 21 patients (aged 23-66 years) examined with gadolinium-enhanced MR imaging were graded for signal intensity on T2-weighted images, contrast material enhancement, shape, and internal septations and were correlated with histopathologic findings. RESULTS Fibroadenomas demonstrated high T2 signal intensity with enhancement (n = 11), low T2 signal intensity with enhancement (n = 3), or low T2 signal intensity without enhancement (n = 9). Low T2 signal intensity and lack of enhancement were associated with more sclerotic stroma and older patient age. Lesion shape was lobular, oval, or round in 19 of 23 fibroadenomas (83%). Internal septations were identified within nine of 14 enhancing fibroadenomas (64%) and appeared to correlate with collagenous bands at histopathologic analysis. CONCLUSIONS Fibroadenomas demonstrate marked histopathologic variability. The resultant variability in the MR appearance limits the ability to distinguish between benign and malignant masses on the basis of signal intensity and enhancement alone. Lobulation and internal septation, which appear to reflect intrinsic growth patterns of fibroadenomas, may provide a more reliable basis for distinction.