A prospective study was completed of 30 shocked patients admitted consecutively to an intensive therapy unit; the majority of the patients had shock of septic origin. Measurements were made of whole blood lactate together with mean arterial pressure, hourly urine volume and core: peripheral temperature difference at the start of treatment and 3 and 24 hours later. Serial lactate measurements were better at predicting outcome than single measurements. However, lactate measurements were much less valuable than serial measurements of the simple haemodynamic variables.