Abstract Tobacco use after cancer diagnosis is associated with adverse cancer outcomes, yet reliable prevalence estimates for this behavior are lacking. We conducted a systematic literature review of the prevalence of current tobacco use among individuals with a history of lung or head/neck cancer (CRD #42012002625). An extensive search of electronic databases (MEDLINE, EMBASE, Cochrane Library, CINAHL, PsycINFO, and Web of Science) identified 7,777 potentially relevant articles published between 1980 and 2014 and 131 of these yielded pertinent information. Aggregating results across heterogeneous study designs and diverse patient samples, the overall mean prevalence rate of current tobacco use (mostly cigarette smoking) was 33.0% (median, 31.0%). Among current tobacco users at cancer diagnosis, the mean prevalence rate of current tobacco use (mostly cigarette smoking) was 53.8% (median, 50.3%). In many cases, an operational definition of “current” tobacco use was absent, and biochemical verification of self-reported smoking status was infrequent. These and other observed methodologic limitations in the assessment and reporting of cancer patients' tobacco use underscore the necessity of uniform tobacco use assessment in future clinical research and cancer care. Cancer Epidemiol Biomarkers Prev; 24(10); 1450–61. ©2015 AACR.