Gastrin stimulates the growth of some human colon adenocarcinomas grown in vitro or as xenografts in nude mice. To evaluate the possibility of elevated plasma gastrin levels in patients with adenomatous polyps or colorectal cancer, we carried out a radioimmunoassay in subjects fasting overnight and undergoing colonoscopy. The study included 190 patients who were divided into three groups: controls (n = 65), those with benign adenomas (n = 63), and those with adenocarcinomas (n = 62). The mean values of plasma gastrin in the cancer group (112.71 +/- 16.65 pg/ml) were significantly higher than those of the control group (40.41 +/- 1.88 pg/ml) as well as those of the polyp group. Mean plasma gastrin values in the polyp group (54.27 +/- 5.29 pg/ml) were also significantly higher than those of the control group. In the cancer group, 32 of 62 patients (51.6%) had gastrin levels greater than the control mean +2 SD, as opposed to only 10 of 63 (15.9%) in the polyp group. The number, size, histologic type, and presence of dysplasia in the polyp group and the location or Dukes' stage in the cancer group had no significant influence on gastrin levels in this study. Preliminary results in cancer patients with elevated preoperative gastrin levels show a postoperative reduction in six of seven patients. The exact cause and role of hypergastrinemia in tumor growth in such patients remains to be determined. Measurements taken both before and after colectomy coupled with a systematic search for specific gastrin receptors would be useful.