Some reports describe an increased risk for cancer in patients with the Peutz-Jeghers syndrome. To characterize occurrences of cancer in a large cohort of patients with the Peutz-Jeghers syndrome. Retrospective cohort study. Tertiary care center. 34 patients with the Peutz-Jeghers syndrome identified from Mayo Clinic records from 1945 to 1996. Cases of cancer documented by chart review and telephone follow-up. 26 cases of noncutaneous cancer developed in 18 of the 34 patients: 10 cases of gastrointestinal cancer and 16 cases of extraintestinal cancer. With the use of SEER (Surveillance, Epidemiology, and End Results) data for comparison, the relative risk for cancer was 18.5 (95% CI, 8.5 to 35.2) in women with the Peutz-Jeghers syndrome and 6.2 (CI, 2.5 to 12.8) in men with the syndrome (P = 0.001). In women, the relative risk for breast and gynecologic cancer was 20.3 (CI, 7.4 to 44.2). The Peutz-Jeghers syndrome is associated with an increased risk for cancer. The relative risk for breast and gynecologic cancers is particularly high.