Twenty one patients with suspected prolactin-secreting microadenoma were evaluated with MRI. MRI is the most sensitive means for detecting focal microadenomas. In these patients who were clinically and endocrinologically considered to harbor a microadenoma, MR detected a focal pituitary signal abnormality in 100% when the patients had not previously taken bromocriptine therapy. On the other hand MR demonstrated a focal abnormality in only 30% of cases when the patients had been on dopamine agonist therapy: the MRI findings in the group of bromocriptine treated patients are not affected by neither the duration and dosage of the therapy or the delay between MR examination and the bromocriptine therapy discontinuation. Among the 9 cases in which MRI did not demonstrate a focal abnormality, MRI was strictly normal in 3 cases only; MR showed a localized expansion of the subarachnoid space in two cases; the pituitary gland was large and round, and had a homogeneous signal in one case; the pituitary gland had a heterogeneous signal in 3 cases. In our study the microadenoma had a high signal intensity on the precontrast T1 weighted sequence in 5 cases. The focal abnormality was not seen on the precontrast MR images in 2 cases. The microadenoma enhanced in 3 cases on the postcontrast three dimensional MR images. This MR technique allows thin section slices (1 mm) and therefore the detection of small focal abnormality of the pituitary gland (2.5 x 3 mm). Thus three points have to the emphasized: a) MRI always detected a microadenoma when the patients had never received a bromocriptine therapy.(ABSTRACT TRUNCATED AT 250 WORDS)