Twelve fibroadenomas were studied in vivo using magnetic resonance (MR) imaging with a cylindrical surface coil and at 0.15 T. On T1-weighted images, most fibroadenomas had an intensity similar to that of normal fibroglandular tissue; areas of inhomogeneity were seen in six. On five of the T2-weighted images, the fibroadenomas gave a signal equal to or slightly greater than that of normal fibroglandular tissues; five showed a marked increase in intensity, and two gave a low signal intensity. Four fibroadenomas were inhomogeneous on both T1- and T2-weighted images, and two additional lesions became inhomogeneous on T2-weighted images. No consistent correlations were found between MR appearances and histopathologic findings in the ribroadenomas. It is concluded that the appearance of fibroadenomas on MR images is sufficiently variable that it would be difficult to differentiate benign from malignant circumscribed tumors on the basis of MR imaging alone.