Interferon treatment means for many patients prolongation of their life. On the other hand, this treatment is associated with many undesirable effects listed below. Symptoms reminding of influenza (fever, articular and muscle pain, fatigue, loss of appetite, gastrointestinal complaints and mild psychic irritability) are frequent. As regards laboratory examinations, interferon alpha treatment is associated with a drop of the number of leucocytes and to a smaller extent also other blood elements and with a rise of liver enzyme levels. Serious and life threatening undesirable effects are fortunately rare. In rare instances cardiovascular complications were observed (impaired rhythm, drop of blood pressure) and sometimes interferon treatment was suspected to be associated with myocardial infarction. Interferon alpha can cause deterioration of already existing autoimmune disease or cause renal failure. In rare instances psychoses induced by interferon alpha were described as well as central and peripheral neurological disorders. As the number of patients treated with interferon alpha is increasing, all doctors treating these patients should know its undesirable effects.