In order to assess the role of immunogenetic factors in the pathogenesis of idiopathic pulmonary fibrosis (IPF), HLA-A, B, C, and DR typing was performed on 20 Caucasian patients with IPF seen from 1978 to 1980 and in 200 healthy blood donors. An increase in the frequency of the B-cell alloantigen, HLA-DR2, was observed; HLA-DR2 was found in 65% of patients with IPF compared with 26% of the control subjects (corrected p value = 0.005, chi-square = 11.476, relative risk = 5.286). Although not significant, increases were observed in HLA-B7 and HLA-A3, antigens known to be in linkage disequilibrium with HLA-DR2. The HLA-DR2 was observed in 12 of 14 patients with “ground glass linear” and “cystic honeycombing” radiographic patterns, whereas it was observed in only 1 of 6 patients with “ascinar nodular” radiographic patterns (p = 0.01). No other correlation was observed between HLA type and clinical, laboratory, or pathologic parameters. We conclude that immunogenetic factors, perhaps genes within the major histocompatibility complex, are relevant to the pathogenesis of IPF.