Tumor scanning with 201Tl-chloride was performed in 15 patients with untreated carcinoma. In 12 of 15 patients 67Ga-citrate scanning was performed to compare the detection rate of the lesions. In 11 of 15 cases, lesions were visualized by 201Tl-chloride, while 9 of 12 cases were positively delineated by 67Ga-citrate. Good results were obtained by 201Tl-chloride in patients with lung and thyroid cancer. The advantages and disadvantages of 201Tl-chloride as a tumor scanning agent are discussed.