The adult respiratory distress syndrome (ARDS) which involves increased protein permeability oedema of the lungs is thought to result, in general, from microvascular injury by activated neutrophils. However, it can also occur during neutropenia. The 1-hour pulmonary accumulation of Gallium (Gal-67, corrected for blood volume, the so-called pulmonary leak index (PLI), is elevated during ARDS in non-neutropenic patients. Whether this is a measure of protein permeability, and truly neutrophil independent, is open to question since intravenous injected Ga-67 may not firmly bind to circulating transferrin but to lactoferrin present in, or derived from, neutrophils. Three patients with circulating white blood cell counts between 0.1 and 0.7 x 109/l in the course of treatment for haematological malignancy were studied: Patients 1 and 2 at the time of ARDS, and Patient 2 also during recovery from ARDS and neutropenia. Patient 3 had normal lung function and a normal chest X-ray, and was studied before and on the sixth day after starting chemotherapy. The Ga-67 PLI was measured after intravenous injection of Ga-67 and Technetium (Tc)-99m-red blood cells. Compared to Patient 3, in whom the PLI was 7.5 before and 10.4 x 10-3/ min on the sixth day after start of treatment, and to controls (n = 12 scheduled for cardiac surgery) with a PLI of 8.2 ± 3.7 (SD) x 10-3/min, the PLI was elevated in Patients 1 and 2 to 20.0 and 52.4 x 10-3/min, respectively, and normalised to 6.6 x 10-3/nmin in Patient 2 in the follow-up study. These preliminary data support the view that the Ga-67 PLI is neutrophil independent, specific for protein permeability in the lungs, and helpful in diagnosing ARDS, even in the presence of neutropenia.