There are large differences in the effectiveness of disease modifying anti-rheumatic drugs(DMARD) from one person to the next. Adverse drug reactions caused by DMARD can also occur to some patients, but do not occur to others. One possible cause of the differences in the effectiveness and adverse drug reactions is genetic variation in how individuals metabolize drugs. The Human Genome Project heralds new opportunities for using information about genetic variation to predict responses to drug therapies, called pharmacogenetics. Based on pharmacogenetics, tailor-made drug therapy is going to be realized. Our recent studies revealed the relationship between genetic polymorphisms of drug metabolizing enzymes and the efficacy of methotrexate or sulfasalazine in patients with rheumatoid arthritis, suggesting pharmacogenetics is applicable to the treatment of rheumatoid arthritis.