The causes of hypothermia in 89 lambs were identified on the basis of history and clinical biochemistry. Excessive heat loss accounted for 24 per cent of the cases, and depressed heat production because of either severe hypoxia during birth, immaturity or starvation accounted for 72 per cent. Exhaustion of energy reserves and hypoglycaemia were marked characteristics of lambs which became hypothermic after 12 hours of age. Most of the lambs were either twins or triplets. The implications of the findings for both the treatment and prevention of hypothermia in newborn lambs are discussed.