We prospectively screened 129 asymptomatic subjects (mean age 64 yr) with flexible sigmoidoscopy. Colonoscopy was performed at a later date, regardless of the sigmoidoscopic result. Our intent was 1) to establish the prevalence of proximal neoplasms in patients with and without hyperplastic polyps within reach of the 60-cm sigmoidoscope and 2) to determine whether a distal (sentinel) hyperplastic polyp predicts the presence of synchronous neoplastic polyps higher up in the colon. Our results show that 15% of asymptomatic adult subjects without polyps on sigmoidoscopy have adenomas in proximal colonic segments that can be diagnosed only by colonoscopy. By comparison, proximal neoplasms were detected in 32% (p less than 0.05) and 37% (p less than 0.05) of patients when hyperplastic or adenomatous polyps, respectively, were present on the sigmoidoscopic examination. This finding suggests that a distal (sentinel) hyperplastic polyp by itself may be a marker for neoplastic polyps in proximal colonic segments. Also, the "index" adenoma and "sentinel" hyperplastic polyp may be equivalent for predicting the presence of proximal neoplasms. The observed detection rates for these polyps were both significantly higher than expected when compared to patients who did not have polyps in the distal colon or rectum. If these results can be confirmed by a larger prospective trial, then full colonoscopy for detection of proximal neoplasms may be indicated when either an index adenoma or sentinel hyperplastic polyp is detected by sigmoidoscopy.