The prevalence and features of malnutrition in COPD patients have been studied extensively in stable conditions but are poorly defined in the presence of acute respiratory failure (ARF). Nutritional status was prospectively assessed, on hospital admission, in 50 consecutive COPD patients presenting with ARF, 27 of them requiring mechanical ventilation (MV). Malnutrition, defined on a multiparameter nutritional index, was observed in 60 percent (30/50) of all patients, and in 39 percent (13/33) of those whose body weight was equal to or above 90 percent ideal body weight (IBW). Malnutrition was more frequent in those patients who required MV than in those who did not (74 percent vs 43 percent, p < 0.05). Subcutaneous fat stores were decreased (triceps skinfold thickness [TSF] < 80 percent pred) in 68 percent of patients, and markedly depleted (TSF < 60 percent pred) in 52 percent of them. The indices of lean body mass, ie, mid-arm muscle circumference (MAMC) and creatinine height index (CHI) were decreased in, respectively, 42 percent and 71 percent of patients, but MAMC was severely depressed (< 60 percent pred) in only 6 percent of them. A severe decrease of prealbumin (< 100 mg/L), retinol-binding-protein (< 20 mg/L), and albumin (< 20 g/L) serum concentrations was observed in, respectively, 22 percent, 28 percent, and 4 percent of patients. These results suggest that an assessment of nutritional status using a multiparameter approach should be systematically performed in COPD patients with ARF, especially in those requiring MV, as malnutrition may have deleterious effects on weaning off MV.