The relative contribution of a meal, exercise and diuretic therapy to within day variation of resistance values from bioelectrical impedance measurements was studied in several groups of patients with severe Chronic Obstructive Pulmonary Disease (COPD). All patients were in a stable clinical condition. Fasting resistance (RES) in the early morning was significantly higher than RES values during the day ((mean +/- SD): 26 +/- 21 Ohms, p < 0.001). Relative to a non-fasting pre-meal value no significant decrease in RES was measured within 4h after ingestion of the meal. RES did not change significantly after a submaximal exercise challenge on the bicycle ergometer. No consistent effect of oral diuretics (40 mg furosemide), was established within 5h after administration. Measurements on two consecutive days at the same time in the nourished state, were highly reproducible: mean change 4 +/- 20 Ohms, NS). We conclude that in patients with stable COPD, within day variation has only a slight impact on the estimation of fat free mass for between-patient comparison, but it is important to take this into account during follow up studies.