The present study was designed to test the possibility that spontaneous regression of hepatocellular tumors might be observed in mice. This problem was studied by sequential liver biopsies in C3H male mice that had been treated with dieldrin (CAS: 60-57-1) as well as in animals treated with N-diethylnitrosamine (CAS: 55-18-5) and in untreated control mice. Adenomas were seen in some animals at the second laparotomy when there had been no tumor at the first laparotomy. In a few mice there was histologic progression from adenoma to carcinoma. A change in predominant cell type in adenomas from clear to basophilic or eosinophilic was also observed in some cases. Additional hepatocellular carcinomas were observed in some animals necropsied at 2 years of age. These observations suggest that spontaneous hepatic tumors and tumors in mice treated with either complete carcinogens or nongenotoxic compounds have a strong tendency to progress. Tumor regression in mice appears to be unusual. No consistent relationship of histologic grade of hepatocarcinoma to the type of chemical employed was observed.