Leflunomide (LEF) is one of the disease-modifying antirheumatic drugs (DMARDs) used in the treatment of rheumatoid and psoriatic arthritis. Results of clinical trials and post-marketing studies documented that LEF is effective in reducing disease activity and improving quality of life. The safety data show no increased risk of adverse events during LEF treatment, in comparison with other DMARDs. The incidence of adverse events during LEF therapy is not significantly different from that observed with methotrexate or sulfasalazine treatment. The most common LEF-associated adverse events include: diarrhoea, elevated liver enzymes, alopecia and rash. Adverse events are usually observed during the first 1-3 months of therapy. Careful monitoring of liver function tests and complete blood cell count is necessary during LEF therapy, especially within the first 6 months of treatment.