Based on the fact that Brazilian ethnic groups are mixed and therefore different from Caucasian, Chinese or japanese populations the authors studied the HLA antigen distribution in 37 myasthenic patients. The control group consisted of 69 healthy individuals of the same population studied in the same laboratory. The antigens HLA B8 showed the highest relative risk for female, younger than 40 years old, with thymic hyperplasia. Also the HLA B8 and A1 showed high frequency for black myasthenic patients.