In view of the evidence for an autoimmune pathogenesis of the Lambert-Eaton myasthenic syndrome, we have sought associations with IgG heavy chain allotypes (Gm) and HLA antigens in 30 patients, of whom 20 had evidence of lung carcinoma (histologically proven small ("oat") cell type in 17). A highly significant overall increase in frequency of Glm(2) (chi 2 = 10.95; p less than 0.001; n = 30) and of HLA-B8 (chi 2 = 19.07; p less than 0.001; n = 23) was observed. These two factors apparently occurred independently of each other. The Glm(2) frequency in 36 non-myasthenic small cell carcinoma cases was the same as in a control panel (n = 167). We conclude that Glm(2) and HLA-B8 both associate with increased susceptibility to the Lambert-Eaton myasthenic syndrome, and suggest that Glm(2) may be in linkage disequilibrium with a limited number of VH genes coding for antibodies to restricted antigenic determinants at the nerve terminals, which may be shared by the carcinoma cells.