Seventy-five patients with hepatocellular carcinoma were treated with proton beams from 1983-1993 at the Proton Medical Research Center, University of Tsukuba. For the purpose of confirming the feasibility of proton therapy for hepatocellular carcinoma, we investigated the influence of proton therapy on liver function and also tried to evaluate the possibility of optimization using dose-volume histogram (DVH) analysis. The results indicated that proton therapy did not cause clinically symptomatic damage in liver function and the only notable change after proton therapy was the transient increase of transminase. DVH analysis showed that this transient increase of transaminase was well correlated to the normal tissue complication probability (NTCP). These results indicate that localized high dose radiation using proton beams is feasible for the treatment of liver cancers and optimization of this treatment may be possible using DVH analysis.