The HLA-A and -B antigen distribution in 132 Hong Kong Chinese patients with Graves' disease, including 24 with thyrotoxic periodic paralysis, was compared with that in 110 controls. The HLA-DR antigen distribution in 68 patients was compared with that in 47 controls. The prevalence of Bw46 was significantly increased in patients with Graves' disease and with periodic paralysis. The prevalence of DRw9 was also slightly increased but not at a statistically significant level. The high prevalence of Bw46 related to patients with early age at onset whereas patients with later age at onset had a significantly increased prevalence of HLA B5. It is suggested that Graves' disease in Chinese is due to two HLA-associated mechanisms in which early-onset disease is associated with Bw46 and late-onset disease with B5.