From 1970 to 1985, 663 patients underwent curative resection of colon and rectal adenocarcinomas. All surgical specimens were examined for tumor "budding," defined as small clusters of undifferentiated cancer cells ahead of the invasive front of the lesion. Patients were divided into two groups according to degree of budding: none or mild (BD-1) and moderate or severe (BD-2). BD-1 occurred in 493 patients (74.4 percent), and BD-2 was found in 170 patients (25.6 percent). More severe budding was associated with worse outcome: 71.1 percent of BD-2 patients had recurrence, compared with 20.0 percent of BD-1 patients (P < 0.005). The five-year survival rate was worse in BD-2 than in BD-1 (22.2 percent vs. 70.7 percent; P < 0.001). The 10-year survival rate was also worse in BD-2 than in BD-1 (13.8 percent vs. 50.6 percent; P < 0.001). The incidence of BD-2 rose with the Dukes stage. However, the five-year survival rate of Dukes B patients with BD-2 lesions was worse than that of Dukes C patients with BD-1 cancers (29.1 percent vs. 66.2 percent; P < 0.001). Moreover, there was no difference in five-year survival among BD-1 patients with either Dukes B or C lesions (68.3 percent vs. 66.2 percent). The presence of more severe budding appears to indicate a vigorous biologic activity of colorectal cancer. Thus, meticulous follow-up--and possibly adjuvant chemotherapy--may be beneficial for patients with marked budding, regardless of their Dukes stage.