Patients with anorexia nervosa (AN) frequently suffer from a mild degree of anemia and from moderate leukopenia on top of their undernourished state and metabolic disarrangements. To evaluate in vitro granulopoiesis and its relationship to cytokine production and undernutrition, we have studied 10 adolescent girls with moderate AN (age range, 13.5-18.0). Study methods included assessment of peripheral blood (PB) granulocyte-macrophage colony- forming cells (GM-CFC) of the patients and age-matched controls, and determination of plasma and conditioned medium (CM) of mononuclear cells levels of IL-1, IL-3, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor (TNF), all of which may play a role in GM- CSF growth regulation. GM-CFC numbers were significantly lower in AN patients compared with the normal controls (13.09 ± 11.15 versus 39.33 ± 26.61 colonies/5 x 105 cells, p < 0.01). No inhibitory effect was found in either plasma or CM of patients with AN. However, when CM were applied to non- recombinant human GM-CSF-stimulated normal bone marrow GM-CFC targets, the number of colonies stimulated by the CM of patients with AN was significantly lower than those stimulated by the CM of the controls (73.5 ± 20.1 versus 113.0 ± 11.6, p < 0.025). GM-CSF concentrations in CM were significantly lower in patients with AN compared with normal controls, but no such differences were found in IL-1, IL-3, IL-6, or TNF concentrations. These results indicate defective in vitro granulopoiesis in AN patients, manifested by a reduction of both GM-CFC and GM-CSF. It has to be determined whether these changes are the result of the basic disease process or are they due to malnutrition.