We analyzed serum triglyceride (TG) levels in 22 chronic myeloid leukemia (CML) patients treated with interferon-alpha (IFN-alpha). Hypertriglyceridemia was present in one-half of patients at diagnosis, and IFN-alpha treatment was associated with a further increase in 90% of the total group of patients. This increase was maximal during the first months of therapy. Four patients (18%) reached levels higher than 1,000 mg/dl. This is the first report describing this secondary effect in CML patients treated with IFN-alpha.