To find and eradicate synchronous neoplasms, colonoscopy was performed before and after resectional surgery in 50 patients with "occluding colorectal cancer," defined as encroachment of the lumen by tumor to a degree that prevented passage of a colonoscope. Synchronous, frequently multiple adenomas were found in 29 (58 percent) of these patients. Three patients (6 percent) had synchronous invasive cancer as well. None of these lesions was detected by intraoperative palpation, even though 46 percent of them measured more than 1.0 cm in diameter. Synchronous neoplasms were found significantly more often in patients with occluding cancer than in patients with non-occluding cancer, investigated concurrently at the same hospital. The former patients appear to be in double jeopardy with respect to synchronous neoplasms, these being more prevalent and less accessible than in patients with non-occluding tumors. Moreover, most of the synchronous lesions are undetectable by palpation. These findings bear out the importance of early postoperative, as well as preoperative, colonoscopy in all patients with occluding colorectal cancer.