Methotrexate's mechanism of action affects both the inflammatory and immunosuppressive aspects of response. Its kinetics are defined and include variable absorption, intracellular metabolism, and both renal and biliary excretion. Methotrexate is clearly effective in the treatment of rheumatoid arthritis and may be able to decrease the rate of formation of new bony erosions. It is also effective in psoriatic arthritis and is being used in a multiplicity of other rheumatic diseases. The most common toxicities ascribed to methotrexate are gastrointestinal (e.g. stomatitis) and central nervous system (e.g. headache, fatigue, malaise). Methotrexate-induced hepatic cirrhosis is less common in rheumatoid arthritis than previously thought, although its occurrence in psoriasis is probably higher than in rheumatoid arthritis. Haematological, renal and pulmonary toxicity occur, but are rare, while teratogenicity is well documented. A new and disturbing adverse event, pseudolymphomas are being reported at present.