Myeloablative conditioning regimens commonly lead to prolonged anorexia and poor oral intake. In a prospective study of 147 patients receiving CY, total body irradiation and allogeneic hematopoietic cells, we determined the extent of decline in oral intake and assessed plasma cytokine levels and development of acute GVHD as explanations for protracted anorexia. For each patient, daily oral caloric intake was expressed as a percent of estimated basal requirements, calculated as basal energy expenditure, through day 20. Oral caloric intake was significantly reduced in 92% of patients and remained low. The nadir in oral intake occurred at days 10-12, when median oral caloric intake was 3% of basal energy requirements. Plasma cytokines known to affect appetite (IL2, IL6, tumor necrosis factor-alpha) were significantly elevated above normal following conditioning therapy (P<0.001 for each cytokine). Acute GVHD did not appear to affect oral intake to transplant day 20 in this cohort of patients; however, plasma levels of IL6 rose steeply before the clinical onset of GVHD. Persistent fever occurred with the greatest frequency in patients with most profound reduction in oral intake. We conclude that prolonged alterations in oral intake following this myeloablative regimen may be related to circulating cytokines known to alter eating behavior.