Introduction: Cigarette smoking continues to decline in the U.S., but cannabis use is increasing. Many people who smoke cigarettes also use cannabis. This study examines the characteristics of persons who co-use and those who do not co-use and the likelihood of quitting cigarettes for callers to Kick It California, a large state tobacco quitline., Methods: Data were examined from Kick It California callers from January 2020 through December 2023 (N=45,151), including those from a subgroup randomly sampled and reached for evaluation at 7 months after quitline enrollment (n=3,545). The rate of cigarette smoking cessation at 7 months after enrollment for people who co-use cannabis was compared with that for people who do not. Analyses started in 2023 and concluded in January 2024., Results: More than a quarter (27.2%) of Kick It California callers co-used cannabis. They were more likely to be male, to be younger, and to have a mental health condition than those who did not. Those who co-use cannabis and those who do not have similar rates of receiving quitline counseling or using Food and Drug Administration-approved cessation aids. Controlled for effects of personal characteristics and use of smoking-cessation services, people who co-use cannabis were less likely to quit cigarette smoking 7 months after enrollment (23.2% vs 28.9%; p<0.001). Among those who co-use, 42.9% intended to quit using cannabis in the next 30 days., Conclusions: A substantial percentage of tobacco quitline callers use cannabis. Those who do co-use quit cigarette smoking at a lower rate than those who do not. Over 40% of people who co-use reported intention to quit cannabis, making tobacco quitlines a rich environment to learn about people who co-use and develop strategies for intervention., (Copyright © 2024 American Journal of Preventive Medicine. Published by Elsevier Inc. All rights reserved.)