This study examined the utility and accuracy of collateral reports of smoking status among 256 abstinent alcoholic smokers (140 males, 116 females) treated for smoking cessation. Data were gathered prospectively from two randomized clinical trials of behavioral smoking cessation treatment conducted in San Diego, CA. The mean age of the participants was 42.0 years (S.D.= 10.0) and 93% were Caucasian. Self-reported smoking status was obtained at posttreatment (1 week after the target quit date) and at 1 year. Collateral reports obtained by telephone and expired air carbon monoxide (CO) levels of < 10 ppm were used to confirm self-reported smoking status. Collateral reports were available for 89.1% of subjects at posttreatment and 90.6% of subjects at 1 year. The smoking abstinence rates were similar when using collateral reports or CO confirmation of smoking status at both time points. Collateral reports refuted self-reported abstinence as often or more than CO levels, and showed relatively high concordance with CO levels. In conclusion, collateral reports have utility and are reasonably accurate for confirming self-reported smoking status in clinical trials of smoking cessation for abstinent alcoholic smokers.