A prospective study of 16 patients with hemoptysis was conducted over a 2 1/2-year period. Technetium-99m sulfur colloid (Tc-SC) and technetium-99m-labeled red blood cells (Tc-RBC) were sequentially used to identify the general area of bleeding. Tc-SC accurately located the bleeding area in 3 patients and Tc-RBC identified the bleeding area in 8; there were no false-positive studies. Tc-SC scans were positive at 6-10 minutes and all patients had positive Tc-RBC studies. In general, the scans accurately located the source of bleeding as determined by bronchoscopy and/or surgery. Because of the small sample, statistically significant differences between both techniques were not ascertained. Both Tc-SC and Tc-RBC may prove to be helpful, minimally invasive imaging procedures for determining the general area of hemorrhage in patients with clinically significant hemoptysis. Although a trend suggests that Tc-RBC is a more sensitive method, further comparison is needed for corroboration.