Hair mercury levels increase with the amount of fish in the diet and the amount of mercury in the fish species consumed. If hair mercury levels in people throughout the world were monitored by a standard analytical procedure, the results would indicate locations where people's body burden of mercury is high enough to be subclinically unhealthy and where controls on environmental emissions might be beneficial. The relationship of hair mercury concentration to the method of sampling and analysis of hair, the analysis of the results, the amount of fish consumed, the country and location from which samples were taken and the age, sex and occupation of the donor is discussed.