DNA ploidy of hepatocellular carcinoma (HCC) was studied in 28 patients using a flow cytometric method. Fourteen patients had two HCCs synchronously, and the remaining 14 had tumor recurrence in the remnant liver 3-41 months after curative resection of primary HCCs. DNA ploidy pattern and histopathologic parameters were compared between the synchronous and metachronous HCCs. Among those with synchronous HCCs, both tumors were diploid in 7 cases and aneuploid in 2 instances. Five patients had HCCs of different DNA ploidy pattern. On the other hand, 5 of 14 patients with metachronous HCCs had a consistent DNA ploidy between primary and recurrent tumors. In 4 cases, the first tumor was diploid whereas the recurrent HCC was aneuploid or tetraploid. In the remaining 5 cases, the primary HCC was aneuploid, but the recurrent tumor was diploid. Assuming that the difference in DNA ploidy pattern indicates a different clonal origin, the current results indicate that at least 36% of synchronous HCCs and 64% of recurrent HCCs develop in a multicentric fashion.