Observation of the hair color in children from 1 month to 5 years old with the unaided eye showed the prevalence of the dark hair shades (M through Y) in children of both sexes in the first half year of their lives. The change toward the prevalence of light hair color shades (A through L) occurred from 9 months to 2 1/2 years old. Between the ages of 3 and 5, the hair became progressively darker. Very light (A-E) and black hair (U-Y) were rare. Red hair occurred in 1 to 2 percent of the study sample. The microscopic examination of selected hairs analyzed for the same child between the ages of 1 month and 5 years found that hair diameter approximately doubled between 1 month and 1 year old. Pigment was abundant in dark hair at 1 month (shade T). The hair became lighter at the ages of 6 and 9 months and changed to shade K at 3 years. At 5 years old, the hair became dark again (shade S). The medulla (the canal of air or liquid-filled cells in the center of the cortex, the main body of the hair) developed at 3 years old. The study was based on a chronological followup of 232 healthy children (114 boys and 118 girls) who lived in Zizkov, a section of Prague with approximately 100,000 inhabitants. The sample surveyed was composed of children born only on Wednesdays. The hair samples were taken at each visit to the health center when each child was 1 month, 3 months, 6 months, 9 months, and 1 year old. Hair samples were collected at 6-month intervals until the age of 5 years. 6 figures