Despite their advantage for obtaining representative samples of adolescents, telephone surveys have been regarded as inferior for collecting data on youth tobacco use because they yield lower estimates than school-based self-administered surveys. Although no gold standard for smoking estimates exists, the lower estimates in telephone surveys have been attributed to underreporting due to youths' concern about parents or others overhearing their responses. Telephone audio computer-assisted self-interviewing (T-ACASI) is a cost-effective method for obtaining a representative sample of youths and provides increased privacy for the respondent. We hypothesized that using T-ACASI would encourage youths to more fully report smoking behavior compared to traditional interviewer-administered telephone methods. Our analysis further assessed whether respondent age, gender, race/ethnicity, and parental attitude toward smoking moderated the relationship between survey mode and smoking reports. Using data from a statewide tobacco use survey that randomly assigned youth respondents to either T-ACASI or interviewer-administered mode, we found youths were more likely to report smoking behaviors in T-ACASI mode and that this was especially true for girls, particularly those who believed their parents would disapprove strongly of their smoking. Findings suggest that traditional telephone surveys may underestimate smoking prevalence in most girls by a factor of two, and that a technique for insuring privacy for these respondents is an important component of effective telephone survey methodology.