To the Editor.— Gilbert et al ( Blood 54(suppl):84, 1979) reported low serum cholesterol levels in 24 patients with polycythemia vera or myeloid metaplasia. We examined the records of 20 patients who had agnogenic myelofibrosis and myeloid metaplasia. There were 12 men and eight women aged 42 to 94 years (x = 70.5±14.5 SD), all without diseases known to alter plasma cholesterol levels. Seventeen patients (11 men and six women) had total plasma cholesterol values significantly lower than those reported in the Framingham study for age-matched men and women. Cholesterol levels for the total 20 population ranged from 61 mg/dL to 256 mg/dL (x = 118.9±40.9 SD). This observed decrease did not seem to be related to levels of peripheral blood erythrocytes, neutrophils, or platelets, nor the degree of splenomegaly. The reason for the low serum cholesterol level in myelofibrosis is unknown.