The results of acid-base and blood gas estimations in arterial and non-arterialized capillary blood have been compared in samples obtained simultaneously from patients presenting with diabetic ketoacidosis. Highly significant correlations were obtained for pH, pCO2 and bicarbonate measurements. Small but significant differences were observed with capillary pH slightly lower and capillary pCO2 and bicarbonate slightly higher than arterial values. These differences were of no clinical significance. Non-arterialized capillary samples are a reliable indicator of acid-base status in this form of metabolic acidosis and are preferable to repeated arterial puncture.