Self-reports of cigarette smoking in a sample of young adults, ages 17 to 21 (n = 359) obtained by telephone were validated by home interview and saliva cotinine measurement. While most subjects were in agreement for their reported behaviors, the telephone methodology underestimated cigarette smoking rates by 3 to 4 per cent. Reported quitting by telephone was an unstable category with 35 per cent of quitters reporting smoking on home interview. Reliance on telephone methods alone may underestimate smoking behavior. Validation of surveys, preferably by biochemical methods, is recommended.