In an attempt to clarify the background of patients with rheumatoid arthritis (RA) who should be treated with salazosulfapyridine (SASP), a tree classification of responders and non-responders to SASP was developed by analyzing 88 RA patients administered with SASP, according to factors such as clinical findings, autoantibody and HLA antigen. Five clinical features were associated with increased efficacy of SASP: male sex, duration of illness of less than 10 years, initial use of SASP, negative RA test and negative antinuclear antibody. In the tree classification for SASP, there were 3 groups of responders: male sex, female sex + initial use of SASP + negative RA test, and female sex + previous treatment with disease-modifying antirheumatic drugs (DMARDs) +negative antinuclear antibody + negative RA test, while there were 5 groups of non-responders: female sex + previous treatment with DMARDs + negative antinuclear antibody + positive RA test with HLA DR9 or A24 + DR9, and female sex + previous treatment with DMARDs +positive antinuclear antibody with HLA A24, DR9 or A24 + DR9. The new tree classification for the efficacy of SASP in the present study facilitated clear distinction of responders and non-responders to the drug.