Studies of liver cancer mortality are subject to confusion attributable to the changes in categories by which liver cancer is identified in successive revisions of the International Classification of Diseases. To determine the effects of these changes, diagnoses of 2,388 cases of primary liver cancer in the years 1973-80 were compared to the underlying causes of death recorded on the death certificates, using data from the National Cancer Institute's Surveillance, Epidemiology, and End Results Program. Results showed that only 53 percent of the deaths were attributed on death certificates to primary liver cancer. In a reverse comparison of 2,977 death certificates from the years 1973-85 with an underlying cause of death of primary liver cancer, 83 percent had been diagnosed as liver cancer. However, among the certificates that specified cancer of the liver, not specified as primary or secondary, as the cause of death, only 40 percent had been diagnosed originally as liver cancer. The mortality of liver cancer can be either underestimated or overestimated depending on which disease classification categories are used.