The frequencies of HLA A, B, C and DR antigens were evaluated in 220 persons from West Germany with inapparent and manifest Borrelia burgdorferi infections. Thirty-seven forest workers showing elevated antibody titres against Borrelia burgdorferi had asymptomatic infection, and are described as stage 0. One hundred and eighty-three patients presented with the clinical stages 1-3 of the infection. Control persons (n = 655) were typed in the same time period and by identical staff. HLA CW3 was present in 36.3% of patients as compared to 23.2% of the controls (RR = 1.88, pcorr = 0.03) and was significantly associated with manifest infection. In addition, the antigen A2 was found slightly but not significantly more frequent in the patients (55.2% vs 44%; pcorr = 0.41). The phenotype combination HLA A2 and Cw3, however, was significantly elevated in patients (24.6% vs 10.8%; pcorr = 0.0005). In contrast to these class 1 antigens, HLA DR3 showed a tendency of negative association with manifest infection. But this finding was not yet found to be significant (15.3% vs 25.3%; RR = 0.53, pcorr = 0.26). The frequency of HLA DR2 showed a constant decrease from stage 0 to stage 3 (inapparent infection to late complications). Using the rank correlation coefficient of Spearman, this association was found to be significant (-1.00, p less than or equal to 0.05). All other tested HLA antigens and antigen combinations showed no significant differences. The data suggest that HLA CW3 may be associated with Borrelia burgdorferi infection, whereas HLA DR2 and DR3 may be associated with less incidence of severe courses and less complications in this disorder.