We studied the frequency of microvessels in T3 N0 M0 colorectal carcinomas from patients with widely different survival times. Microvessels (<50 microm diameter) were enhanced by immunostaining with antibody to factor VIII-related antigen and counted in 40x high-power fields in sections of resected carcinomas from 9 patients who died of disease in 24 months or less (short-term survivors) and 13 who had no evidence of disease at 109 months or longer (long-term survivors). The means of the 10 highest counts for each case were compared between the long- and short-term survivor groups. The mean +/- SD microvessel count was 25.4 +/- 6.5 for the short-term survivors and 30.3 +/- 6.4 for the long-term survivors. Median counts were 27.2 and 29.4, respectively. The distribution of microvessel counts was skewed toward higher counts in the long-term survivors. There was no correlation between microvessel counts and tumor site, size, or grade; lymphovascular invasion; or the presence of a mucinous component. Although there was a trend toward a higher frequency of microvessels in patients with longer survival, it is unlikely that microvessel count is an independent prognostic indicator for patients with T3 N0 M0 colorectal carcinoma because there is only a small difference in microvessel frequency between patients with widely different survival times.