The authors describe the diagnostic possibilities of Thallium 201 in cerebral lesions of various kinds. Two radiopharmaceuticals were used in 124 patients: thallium 201 citrate and technetium-99m pertechnetate labelled. When 201Tl was used, all cases of definite cerebral lesion gave a positive result within 10 minutes of injection. In 85.4% the definition was better, and small metastases which were invisible with 99mTc could be demonstrated. More than two-thirds of the time, neoplastic lesions were better defined and distinguished from adjacent normal areas of hyperactivity. 201Tl promises greater diagnostic usefulness with an examination time of only 30 minutes.