The relationship of tobacco use with risk of primary liver cancer was investigated using data from a 26-year mortality follow-up of nearly 250,000 US veterans, mostly from World War I. Significantly increased risks for liver cancer (289 deaths) were associated with most forms of tobacco use, including pipe and cigar smoking. Elevated relative risks (RRs) were seen for current cigarette smokers (RR = 2.4; 95 percent confidence interval [CI] 1.6-3.5) and former cigarette smokers (RR = 1.9, 1.2-2.9). A strong dose-response relationship (P less than 0.001) was found for cigarette smoking, with smokers of 40 or more cigarettes per day having almost a fourfold risk (RR = 3.8, 1.9-8.0). Risks were also found to increase significantly with years of cigarette use and with earlier age at the start of cigarette smoking. These results are consistent with those of other cohort and case-control studies, suggesting that cigarette smoking may be related to the risk of liver cancer.