Background: Interest in the use of cannabis-based products, including marijuana (products containing ≥0.3% tetrahydrocannabinol [THC]) and cannabidiol (CBD; containing < 0.3% THC) for cancer symptom management, is increasing. Preliminary studies suggest that marijuana and CBD products are effective in alleviating common cancer- or treatment-related symptoms. Few studies have reported on the magnitude of cannabis use among cancer patients, but data remain limited to states with legalized non-medical (i.e., recreational) use. Further, none have assessed the prevalence of CBD use in cancer patients. The reasons for cannabis-based product use and patient attitudes towards discussing use with their physicians are not well understood. Methods: We report preliminary results from an ongoing anonymous cross-sectional study of cannabis use among patients attending 8 clinics at the Ohio State University Comprehensive Cancer Center. Eligible patients were diagnosed with invasive cancer and treated in the past 12 months. Between July 2021 and January 2022, 771 patients were contacted and 639 (82.9%) agreed to participate. Participants answered a validated, extensive cannabis-focused questionnaire and were remunerated with a gift card. We report here frequencies and prevalence proportions of cannabis-based product use, and cannabis-related behaviors and attitudes on reporting use with cancer providers. Missing values were included in the denominator of prevalence calculations, thereby making estimates conservative. Results: Participants were equally distributed by sex, White (86.4%), and resided within Ohio (90.3%). Among 433 patients who reported age, the median was 47 years (IQR 38-53). The prevalence of current cannabis-based product use was 15.2% (97/639), with overall prevalence of 6.9% for exclusive marijuana use, 1.7% exclusive CBD use, and 5.2% poly-use. Among current users, patients primarily reported inhaling or ingesting cannabis products; fewer reported topical or other applications. The top 5 reported reasons for cannabis use were to treat stress, improve sleep, reduce pain, improve appetite, and to treat depression. Among current users, 66% reported that their cancer care team was aware of their cannabis use, 17.5% reported none of their healthcare providers were aware, and 15.4% reported a medical professional unaffiliated with their cancer care was aware. A large proportion (86.6%) felt comfortable discussing cannabis with their treating physician. Conclusions: Our results indicate current cannabis-based product use for symptom relief among a sizeable minority of adults with cancer in a state where only medicinal marijuana is legalized. Further research is needed to characterize provider perspectives and communication about reported cannabis use among their patients.