Sixteen patients with polycythaemia vera or essential thrombocythaemia were treated with interferon-alpha in order to normalize elevated platelets. Patients were followed for 6 months and the frequency and intensity of symptoms and side effects were recorded before and during the study period by the patients and by the doctor. Health-related quality of life was also assessed. The most frequently reported pretreatment symptoms were fatigue, headache and muscle pain. The intensity of fatigue initially increased during treatment and there was no relief of any of the three most frequent symptoms during the treatment period. Common interferon-related symptoms such as fever and chills were most frequently reported after one week. After one month of treatment, symptoms related to the gastrointestinal tract reached a peak. Two patients discontinued treatment during the study period. Another patient suffered severe depression after the study period when still on interferon. There was no difference between the frequency of symptoms recorded by the doctor and that reported by the patients.