METHODS. The expression of ABH blood group carbohydrate antigens was examined histochemically in tumors and adjacent nontumorous tissues of 89 cases of human lung carcinoma in which nontumorous tissues expressed blood group carbohydrate antigens compatible with the erythrocyte blood group types. RESULTS AND CONCLUSIONS. Loss of ABH blood group antigens in lung carcinomas correlated with their metastatic potential, especially with the recurrence of hematogenous metastasis. Consequently, patients with tumors that retained compatible ABH blood group antigens showed a better prognosis than patients with tumors of altered antigenic profiles. The loss of blood group B antigen more significantly affected both the hematogenous metastasis and prognosis than that of A and H antigens.