Cigarette smoking initiation greatly influences smoking prevalence in the United States. To understand better the initiation of cigarette smoking, we estimated the age-specific incidence of cigarette smoking initiation in relation to race/ethnicity, sex, and educational attainment, using the reported age at smoking onset for 18- to 35-year-old respondents in the 1987 National Health Interview Survey (N = 14764) and the Hispanic Health and Nutrition Examination Survey (N = 3123) conducted during 1982 to 1984. Among white, black, and Hispanic respondents the incidence of smoking initiation increased rapidly after 11 years of age, reaching a peak in groups 17 to 19 years of age, rapidly declining in groups through age 25 years, and gradually declining thereafter. Age-specific smoking initiation rates were generally lower among black than white respondents, similar between white and Hispanic respondents, and appreciably higher among black and Hispanic men than women. Compared with persons who graduated from high school, persons with less than high school education were consistently more likely to start smoking cigarettes during childhood and adolescence. These data indicate that age and educational attainment are the factors most consistently associated with cigarette smoking initiation among all race/ethnic groups in the United States. These data also emphasize the need for smoking-prevention education beginning at an early age, particularly among persons of low socioeconomic status.