The overall extravasation rate of albumin, TER (i.e. the fraction of the intravascular albumin mass (IVM) passing into, and during steady state returning from, the extravascular space per unit time) was determined from the disappearance of i.v. injected radioiodinated serum albumin in seven patients with severe chronic obstructive lung disease (COLD) and in seven normal controls. Arterial oxygen tension in patients with COLD was median 60 mmHg (range 47-80, normal greater than 75 mmHg), vital capacity was on the average 55% of expected normal value (median 1.80 litre, range 1.45-1.95), and forced expired volume in first sec was decreased to 21% of expected normal value (median 0.55 litre, range 0.40-0.70). Right-heart catheterization revealed pulmonary hypertension in all but one patient. TER in patients with COLD was median 6.1% IVM/h (range 3.5-10.1) as compared to that of normal controls 6.0% IVM/h (range 4.3-7.4), indicating that no significant change in microvascular leakiness to albumin could be found in patients with COLD. Thus, the results bring no support to a generally increased microvascular permeability to proteins in patients with COLD.