Invasive colorectal carcinomas (CRCs) with invasion confined to the lamina propria (LP) [intramucosal carcinoma (IMC)] lack access to lymphatics and therefore have no potential for metastases and local intervention (usually polypectomy) should be adequate treatment. For this reason, they are classified as "Tis" in the TNM system. It is believed that carcinomas invading the submucosa with unfavorable histology (tumors at/near the margin, and/or vascular invasion, and/or poor differentiation) require additional intervention after polypectomy, whereas those with favorable histology can be safely treated endoscopically. However, there are few data on poorly differentiated (PD) carcinomas showing invasion confined to the LP. Polypectomy is theoretically curative but in practice this has not been well demonstrated. Thus, the clinicopathologic features of 15 cases of PD CRCs with invasion limited to the LP on initial biopsies were studied to determine the best course of management for this rare subset of carcinomas. A computer search and histologic review of cases seen at Johns Hopkins Hospital was performed. Fifteen cases of PD CRC with invasion limited to the LP were identified. The clinicopathologic features of these tumors were reviewed. All 15 cases showed PD IMC with single cells infiltrating only the LP. Patients were 38 to 79 years (median, 62) of age with a male predominance (M:F=4:1). Three cases had signet ring cell differentiation, 1 had focal small cell features, and another had focal squamous differentiation. Fourteen of the cases were associated with background adenomas or adenomalike lesions including: 7 involving tubulovillous or villous adenomas, 6 involving tubular adenomas, 1 involving dysplasia associated with chronic inflammatory bowel disease. Nine of the lesions had surrounding high-grade dysplasia. One case showed no background dysplasia or adenoma. One patient was lost to follow-up and the remaining 14 were followed for 1 to 96 months (mean, 21.3 mo; median, 13 mo). Seven patients had no residual disease on follow-up colonoscopy, and no resection was performed. The remaining 7 patients were treated with partial colectomy (6) or low anterior resection (1), and of these, 5 had no infiltrating carcinoma and negative lymph nodes. One patient had a separate large colorectal (T3) carcinoma with 8/10 positive regional lymph nodes; the IMC seen on biopsy was presumably a metastasis as it was unassociated with an in situ component. Finally, the resected rectum from which an IMC had been previously detected had no residual invasive carcinoma, but the anal skin was involved by Paget disease. Thus, of the 15 cases of PD CRCs limited to the LP, 1 was a metastasis from a separate CRC and another had associated Paget disease of the anal skin. As such, even in the setting of PD carcinomas, no metastatic disease was seen arising from any of the cases that were confirmed as early primary lesions. These preliminary findings suggest that patients with isolated intramucosal PD CRCs may be managed endoscopically.