We examined electronic cigarette (EC) use, correlates of use, and associated changes in smoking behavior among smokers with serious mental illness in a clinical trial.Adult smokers were recruited during acute psychiatric hospitalization (N = 956, 73% enrollment among approached smokers) in the San Francisco Bay Area between 2009-2013. At baseline, participants averaged 17 (SD = 10) cigarettes per day for 19 (SD = 14) years; 24% intended to quit smoking in the next month. Analyses examined frequency and correlates of EC use reported over the 18-month trial and changes in smoking behavior by EC use status.EC use was 11% overall, and by year of enrollment, increased from 0% in 2009 to 25% in 2013. In multiple logistic regression, the likelihood of EC use was significantly greater with each additional year of recruitment, for those aged 18-26, and for those in the preparation versus precontemplation stage of change, and unlikely among Hispanic participants. EC use was unrelated to gender, psychiatric diagnosis, and measures of tobacco dependence at baseline. Further, over the 18-month trial, EC use was not associated with changes in smoking status or, among continued smokers, with reductions in cigarettes per day.Within a clinical trial with smokers with serious mental illness, EC use increased over time, particularly among younger adults and those intending to quit tobacco. EC use was unrelated to changes in smoking. The findings are of clinical interest and warrant further study.