The age of 798 male coal and uranium miners and a few children was estimated based on the maximum hair medulla diameter and other factors such as ratio of medulla diameter to hair diameter, opacity of the medulla, or lucidity of the cortex. Ages of adults ranged from 19 to 70 years and about 50 percent were white and the other were Navajo Indians. Photographs of hair shafts show that the maximum hair medulla diameter is generally small in the child, larger in middle age, and largest in old age. They also suggest that there may be a change in maximum hair diameter and in the ratio of medulla diameter to hair diameter accompanying age changes. Of the 798 age estimates made from the hair samples, 47.8 percent were within plus or minus 5 years of the actual age and 70.2 percent were within plus or minus 10 years. Estimates were made with equal accuracy for Indians and for whites. These observations are considered possibly useful in the study of aging processes in man or in criminological determinations of age.