Arterial blood samples from 202 critically ill patients were studied. Lactate concentration, PO2, PCO2, pH, oxygen saturation, total CO2, and toe temperature were measured. Base excess, buffer base, actual bicarbonate, and standard bicarbonate were calculated. Analysis of the initial data obtained from each patient revealed that lactate concentration alone could not be used to predict survival. Elevations in lactate concentrations were observed in all types of acid-base disturbances. Survival was lowest (30%) for patients in metabolic acidosis with respiratory compensation, mean lactate concentration 10.5 mEq/l (mmol/l), and highest (65%) for patients in uncompensated respiratory alkalosis, mean lactate concentration, 2.57 mEq/l (mmol/l). Knowing the type of acid-base disturbance is necessary to evaluate the significance of elevated blood lactate values.