Although the prevalence of smoking is lower among Hispanics than among the general population, smoking still levies a heavy public health burden on this underserved group. The current study, Adiós al Fumar (Goodbye to Smoking), was designed to increase the reach of the Spanish-language smoking cessation counseling service provided by the National Cancer Institute's Cancer Information Service (CIS) and to evaluate the efficacy of a culturally sensitive, proactive, behavioral treatment program among Spanish-speaking smokers. Adiós was a 2-group randomized clinical trial evaluating a telephone-based smoking cessation intervention. Spanish-speaking smokers (N = 297) were randomized to receive either standard counseling or enhanced counseling (EC). Paid media was used to increase the reach of the Spanish-language smoking cessation services offered by the CIS. The Adiós sample was of very low socioeconomic status (SES), and more than 90% were immigrants. Calls to the CIS requesting smoking cessation help in Spanish increased from 0.39 calls to 17.8 calls per month. The unadjusted effect of EC only approached significance (OR = 2.4, P = .077), but became significant after controlling for demographic and tobacco-related variables (OR = 3.8, P = .048). Adiós al Fumar demonstrated that it is possible to reach, retain, and deliver an adequate dose of treatment to a very low SES population that has traditionally been viewed as difficult to reach and hard to follow. Moreover, the findings suggest that a proactive, telephone-counseling program, based on the Treating Tobacco Use and Dependence Clinical Practice Guideline and adapted to be culturally appropriate for Hispanics, is effective. Cancer 2007. (c) 2006 American Cancer Society.