ABSTRACT: Background:In the United States, young men who have sex with men (YMSM) experience a disproportionate burden of HIV and sexually transmitted infections (STIs). Mobile health (mHealth) interventions, including those that incorporate elements of games ("gamification"), have the potential to improve YMSM engagement in desirable sexual health services and behaviors. Gamification leverages theory and tools from behavioral science to motivate people to engage in a behavior in a context of fun. The objective of the study was to determine whether an intervention using gamification is acceptable to YMSM in California and potentially increases repeat HIV screening. Methods:Eligible YMSM were: (I) 18-26 years, (II) born as and/or self-identified as male, (III) reported male sexual partners, and (IV) lived in a zip code adjacent to one of the two study clinics in Oakland and Hollywood, California. The gamification intervention, Stick To It, had four components: (I) recruitment (clinic-based and online), (II) online enrollment, (III) online activities, and (IV) 'real-world' activities at the clinic. Participants earned points through online activities that could be redeemed for a chance to win prizes during HIV/STI screening and care visits. The primary outcome was intervention acceptability measured with participant engagement data and in-depth interviews. The secondary outcome was the intervention's preliminary effectiveness on repeat HIV screening within 6 months, restricted to the subset of men who provided consent for review of medical records and who had ?6 months of follow-up. Outcomes were compared to a historical control group of similar YMSM who attended study clinics in the 12 months prior to intervention implementation. Results:Overall, 166 of 313 (53%) eligible YMSM registered. After registration, 93 (56%) participants completed enrollment and 31 (19%) completed ?1 online activity in the subsequent 6 months. Points were redeemed in clinic by 11% of the 166 users (27% and 5% of those who enrolled in the clinic and online, respectively). Despite moderate engagement, participants provided a positive assessment of the program in interviews, reporting that the inclusion of game elements was motivating. The analysis of repeat HIV testing was assessed among 31 YMSM who consented to medical record review and who had ?6 months of follow-up. During follow-up, 15 (48%) received ?2 HIV tests compared to 157 (30%) of a historical comparison group of 517 similar YMSM who lived in the same zip codes and who received care at the same clinics before the intervention (OR =2.15, 95% CI: 1.03-4.47, P=0.04). Conclusions:Engagement in the intervention was modest, with YMSM who enrolled in a clinic more actively engaged than YMSM who enrolled online. Among the subset of participants recruited in the clinic, repeat HIV screening was higher than a comparison group of similar YMSM attending the same clinic in the prior year.