Magnetic resonance imaging is a method of choice for the diagnosis of microprolactinomas. Using a Phillips Gyroscan NT 15 (1.5 T), 42 patients whose clinical history and serum prolactin levels were compatible with the diagnosis of prolactinomas were examined. Four patients (3 male and 1 female) with macroadenoma were investigated and excluded, while microprolactinoma was found in the other 38 (32 female and 6 male) patients. Imaging features were similar in all microprolactinomas--hypointensity up to 10 mm in size in T1 weighted images, only sometimes, in the case of a cystic or a hemorrhagic component, also registered in T2 weighted images. Deviation of the pituitary stalk was present in 19 of the 38 patients (16 with microprolactinoma, 3 without any detectable lesion). Convex upper limit of the gland was demonstrated in 17 subjects (16 microprolactinomas, 1 without detectable lesion). Pituitary adenoma is not the only cause of raised serum prolactin level. This can be a consequence of medication, spinal cord tumor, compression of the pituitary stalk, hypothyroidism or a lesion in dermatomes T3 to T5 (mammary gland level). Results from imaging must always be assessed in the light of the patient's clinical history and biochemical parameters.