It is known that pulmonary microcirculation rheology is partly affected by plasma levels of lipoproteins, but only a few data are available for humans. Therefore, in a sample of 30 normal volunteers and 90 patients with various types of primary hyperlipoproteinaemia, the plasma levels of total cholesterol (Chol), low density cholesterol (LDL), the high density cholesterol (HDL), triglyceride (Tg) and fibrinogen (Fib) were measured in conjunction with determinations of plasma viscosity (PV) and the pulmonary capillary red cell volume (RCVc). RCVc was estimated from measurements of the vascular component of the single-breath-diffusing lung capacity for carbon monoxide, using our own modification of the Roughton-Forster's method. By stepwise regression analysis, the variation in RCVc was almost completely accounted for (r2 = 0.87) by variations in PV, Chol, Tg and the anthropometric confounding factors. The proposed explanations for increased pulmonary capillary red cell mass (up to 151% of the predicted value) in hyperlipidaemic patients included the hypothesis of increased pulmonary microhaematocrit, which agrees with the observed in-vitro lipoprotein-dependent increase in erythrocyte aggregability.