We performed a long-term prospective study on venous invasion of colorectal cancer. The degree of venous invasion was divided into four stages (V0 through V3). Venous invasion was classified into three types by location (Vx, Vy, and Vz). Hepatic metastasis occurred in 27%, 33%, and 20% of patients with V2, V3, and Vz tumors, respectively. Local recurrence occurred in 33% and 15% of the V3 and Vz groups, respectively. However, there were no significant differences among the groups in terms of the rate of pulmonary metastasis. The 6-year survival rate for Dukes' stage B tumors was 94%, 88%, and 74% in the V0, V1, and V2 groups, respectively. There was a significant difference in the survival rate between patients with V0 tumors and V2 and V3 tumors. However, no significant difference was noted in the location. In Dukes' stage C tumors, on the other hand, the survival rate was 77%, 56%, and 44% in the V1, V2, and V3 groups, respectively. Also, it was 85%, 73%, and 45% in Vx, Vy, and Vz cases, respectively. Significant differences were noted between V1 and V3 (or V2), and between Vz and Vx (or Vy). It appears that the degree and location of venous invasion influence not only hepatic metastasis, local recurrence, and survival rates but also have prognostic value.