Turcot's syndrome is a rare, genetically transmittable disease in which patients with colonic polyposis (possibly complicated by the progression to adenocarcinoma) have malignant central nervous system neoplasms. Dominant, recessive, and sporadic cases have been described. A 26-year-old man is reported with no relevant family history who had intermittent abdominal discomfort in 1986. Sigmoidoscopy revealed numerous polyps, several of which showed carcinomatous change. Dukes' Stage C colorectal carcinoma was diagnosed. Treatment consisted of total colectomy with construction of a Koch's pouch. He remained well for 3 years until onset of headache, nausea, and vomiting. Computed tomographic scan disclosed a large, circumscribed, enhancing, right frontoparietal mass. After craniotomy and partial resection, histologic review disclosed anaplastic astrocytoma. He received cranial radiation therapy, 6000 cGy, by parallel opposed ports to the tumor bed, and carmustine 200 mg/m2 intravenously every 8 weeks. Flow cytometric DNA analysis was done on the paraffin-embedded archival material from the patient's normal colon, colonic adenocarcinoma, and anaplastic astrocytoma. DNA histograms revealed diploid distributions in all three samples. The G2/M fraction of the astrocytoma was elevated at 16%, and the S-phase fraction of the colonic adenocarcinoma was 19.4%.