Eleven cases of green hair were collected and hair tips were analyzed in the electron microscope by energy-dispersive x-ray microanalysis. In all cases, remarkable copper concentrations could be demonstrated (0.2-1.8% w/w). In normal hair the concentration of copper was below the detection limit (0.02% w/w). Analyses of hair cross sections showed a concentration gradient from the periphery toward the center, the periphery having higher values. This observation is in agreement with suspected contamination of the hair with copper from extraneous sources: analysis of the tapwater in patients' homes showed elevated copper concentrations. In addition, the water showed aggressive properties (pH outside the recommended range, high nitrate concentrations, heavily chlorinated). Investigations by transmission electron microscopy showed damage in the endocuticula. Experimental studies of normal unaffected hair fibers showed that treatment with a copper salt alone could result in binding of copper to the hair but that the extent of copper binding could be greatly increased by damaging the cuticula with chlorinated water. It is suggested that aggressive water attacks copper tubings leading to increased copper concentrations in tapwater and causes cuticular damage, which facilitates entry of copper into the hair.