The etiology of Bell's palsy (BP) is still unknown, but infectious, immunological and genetic factors have been suggested to play a role in the pathogenesis of the disease. We analyzed blood samples of 92 Mexican Mestizo patients diagnosed as having BP according to established international criteria, and the results were compared to a group of apparently healthy controls of the same ethnic origin. HLA class I (A, B, C) and Class II (DR, DQ) products of the major histocompatibility complex (MHC), and the percentages of CD3, CD4 and CD8 T-cell subsets were investigated. The number of family antecedents was surprisingly high (46%), supporting a genetic basis. There was a slight increase of DRw13, suggesting a possible susceptibility class II-linked gene. A significant decrease of DR4 (pc = 0.001) was detected, which may indicate the existence of a resistance DR-linked gene. Thus, a non DR4 carrier may be in high risk of expressing BP. In the acute phase of the disease, the T-cell subsets showed a decrease in CD3 and CD4 cells when compared to controls. CD8 cells were increased in the same stage. A transient T-cell imbalance was thus observed which recovered in the convalescent phase. None of the patients with CD4 lower than 40% were DR4, suggesting that the DR-linked resistance gene may predispose to the T-cell defect.