DNA content was measured by flow cytometry using paraffin-embedded material from 148 primary rectal cancers and 10 distant metastatic lesions. DNA ploidy pattern was classified into the following three groups in terms of the DNA index: 1.00 was defined as the diploid group (DP), 1.00 < DNA index < 1.60 as the low-aneuploid group (LAP) and > or = 1.60 as the high-aneuploid group (HAP). Primary carcinomas were DP in 26.4%, LAP in 36.5% and HAP in 37.1%. The DNA ploidy pattern of the primary tumor correlated well with clinicopathological findings such as depth of invasion, lymphatic invasion, pathological stage, metastasis to distant organs and curability of the tumor. In patients with HAP tumor after curative operation, the recurrence rate (21.6%) in distant organs was significantly higher than those with DP tumor (2.8%) and LAP tumor (6.4%) [p < 0.05]. Two LAP patients and 8 HAP patients with distant metastatic disease had the same DNA ploidy pattern HAP in the metastatic lesions. These data indicate that tumor DNA ploidy patterns classified into three groups in rectal cancer may play an important role in predicting prognosis, including distant metastasis.