This is a review of the data on the safety and tolerance of interferon alfa-2a in the treatment of cancer patients and is particularly aimed at patient care. Since 1981, interferon alfa-2a prepared from human sources has been administered to over 2500 cancer patients and 2500 patients with viral diseases. Adverse effects have been invariably produced following parenteral injections of > 3 × 106 IU. These were mainly flu-like symptoms (> 90%), fatigue (90%), gastrointestinal (85%), central nervous system (65%) and musculoskeletal (60%) disorders. Laboratory abnormalities, though common (> 75%) were rarely dose limiting. Their severity can be reduced by using dose schedules which promote tachyphylaxis, co-medication with paracetamol and evening administration. Fatigue is best controlled by careful dose attenuation and occasional therapy rest periods. At dosages of 3 × 106-18 × 106 IU/injection, interferon alfa-2a therapy can be safely managed on an out-patient basis, requiring minimal or no hospitalization. The frequency and low titre of antibody development to interferon alfa-2a indicates that it is a weak antigen and is suitable for long-term therapeutic application.