Few studies have examined adolescent reporting accuracy for secondhand smoke exposure (SHSe), and never for youth with cancer. SHSe reporting from adolescents being treated for cancer (Mage=14.92 years, SD=1.67) was examined against parent/guardian reports and urine cotinine among 42 adolescent–parent dyads. Number of days in hospital-based lodgings prior to assessment emerged as the strongest predictor of urine cotinine (β=−0.46, p=0.003) and adolescent SHSe reporting significantly predicted urine cotinine (β=0.37, p=0.011) beyond relevant demographic and contextual variables (overall R2=0.40, F(6, 35)=3.90, p=0.004). Findings support adolescents as accurate reporters of discrete SHSe occurrences.