Seven new cases of Menkes's kinky hair syndrome are described from five families. These patients were seen in a period of 3 years in Melbourne and the frequency of the disease is estimated to be 1 in 35,000 live births. Seven other affected males were present in these families and each pedigree was compatible with X-linked inherintance. Hypothermia was noted to be an important clinical feature which has escaped previous attention. Widespread arterial tortuosity and variation in the lumen of arteries has been demonstrated by arteriography and by microscopic examination at necropsy. Fragmentation of the internal elastic lamina is seen in the arterial walls with thickening of the intima in the most severely affected vessels. This process can lead to complete obliteration of major arteries. Involvement of cerebral arteries is the probable cause of the gliosis and cystic degeneration seen in the brain. Metachromasia was observed in fibroblasts cultured from patients and from heterozygous females. Low levels of serum copper and ceruloplasmin were found in all patients studied and a defect in the intestinal absorption of copper has been demonstrated. This appears to be the basic defect in the disease and copper deficiency provides an adequate explanation of all the features of the disease. Parenteral administration of copper may prove therapeutically effective. These findings suggest many new lines of research on copper metabolism and trace metal deficiency.