Blood flow in 97 human tumor nodules, of which 31 were lymphomas, 31 were anaplastic carcinomas, and 35 were differentiated cancers, was measured using the 133Xe clearance method. The lymphomas showed statistically higher blood flow [34.6 +/- 21 (S.D.) ml/min/100 g] than did the anaplastic carcinomas (15.4 +/- 11.4 ml/min/100 g; p less than 0.001) and the differentiated cancers (22.8 +/- 14.9 ml/min/100 g; p less than 0.05). The size of the tumors did not correlate with the blood flow. In the group of differentiated cancers, the blood flow in nodules in areas of earlier irradiation was lower (9.0 +/- 6.3 ml/min/100 g) than in nodules in intact regions (25.0 +/- 14.5 ml/min/100 g; p less than 0.05). Nodules in cicatricial areas after surgical operation had lower blood flow than did nodules in the intact areas, but the difference is not statistically significant. It is obvious that most human tumors have a considerably lower blood flow than what one would expect to find in the surrounding normal tissue.