Of the original Peutz-Jeghers families reported by Jeghers, the "Harrisburg Family" has now been followed for 49 yr. Their 12 affected family members comprise the largest Peutz-Jeghers kindred reported. The course of this family illustrates that Peutz-Jeghers syndrome is not a benign disease. One family member developed a duodenal carcinoma in a hamartoma with adenomatous changes; this progression in the duodenum has not previously been reported. Ten patients underwent 75 polypectomies. One patient developed short bowel syndrome. Three patients died in young adulthood. The development of gastrointestinal malignancy in 2 of 12 affected patients suggests that Peutz-Jeghers syndrome may be a premalignant condition. Consequently, even asymptomatic gastric, duodenal, and colonic polyps should be removed endoscopically. If surgical intervention is necessary, intraoperative endoscopy with polypectomy may prevent the development of a short bowel syndrome. Colonoscopic screening of patients and their family members may be beneficial and surveillance for extraintestinal malignancy appears to be warranted.