In a 36 years old male and a 56 years old female myasthenic patient thymectomy was performed several years ago. In the male patient 5, and in the female patient 7 years after the operation rheumatoid arthritis developed. The rheumatoid arthritis in the male patient was seropositive with marginal erosions in the carpometacarpal and in the tarsometatarsal joints. The female patient had no joint destruction and was seronegative as well. The female patient had also an abnormal ratio between the separated CD4 and CD8 T-cells. The major histocompatibility complex determined by the serological methods revealed a haplotype of HLA-A1-B8-D3-DQ2, which is typically due to the myasthenic disease. In the female patient the molecular analysis of the HLA-D region showed a HLA-DRB1* 0401 allele, which is frequently associated with rheumatoid arthritis. The male patient had no allelic variant which could be related to his chronic disabilitating joint disease.