A critical analysis of the largest reported mass fecal occult blood screening program in the United States. Fecal occult blood testing for the detection of colon cancer remains controversial. We performed a mass screening program from January 24, 1988, to February 19, 1988, with intensive media promotion, including 121 minutes of televised air time. A total of 5,000 primary practitioners were notified by mail. Hemoccult-II tests were distributed to 156,000 individuals; 55,051 (35%) were returned. Ninety-five percent of the respondents were informed of the program by television. A total of 3,375 persons (6%) tested positive for fecal occult blood; of these, 2,469 (73%) informed the center that they saw their physician to initiate a work-up. Information from physicians regarding work-ups was returned on only 1,356 (55%) patients. Diagnostic tests numbered 2,227 (1.6 tests per patient). However, 5% had no testing, 16% had a repeat Hemoccult only, and 35% had neither a barium enema nor colonoscopy performed. Thirty-six colorectal cancers and 212 polyps were identified. The predictive value (i.e., number of cancers per number of patients who tested positive) increased directly by decade. Thirty-three of 36 patients (92%) with cancer underwent either a barium enema or colonoscopy versus only 185 of 438 (42%) patients with a "negative" work-up. Cancers found were carcinoma in situ in 10 patients (29%), Dukes A in 12 (35%), Dukes B in 4 (12%), and Dukes C in 8 (24%); distant metastases were not found in any participant. Thirty-six percent of the tumors were located in either the right or transverse colon. We conclude that: (1) Screening identified early cancers. All were potentially curable and 64% were limited to the bowel wall. (2) Massive Hemoccult distribution was possible over a short interval, but patient and physician compliance was disturbingly low. (3) Total colonic evaluation is mandatory, since at least 36% of tumors were beyond the reach of the flexible sigmoidoscope. (4) Many work-ups were unnecessary (repeat Hemoccults) or inadequate, indicating a need for physician education. 