Fewer than 1 in 10 middle-aged US adults get the recommended amount of aerobic physical activity (PA), greatly increasing their chances of heart disease and stroke. There are over 50 million adults with fitness center memberships, which are used only once per month and, after joining new programs, people quickly stop coming. While many barriers to PA have been cited (e.g., time, cost), we believe that a key, modifiable, and underappreciated barrier is that most adults simply do not enjoy physical activity enough to do it regularly. Studies consistently observe that people who enjoy physical activity more are more likely to be more active in the future. Despite this consistent observation, we are not aware of studies that ask “Would an adult PA program designed to maximize enjoyment increase PA adherence and fitness?”. In 2015, our team set out to design an exercise program for adults that maximized enjoyment. Taking inspiration from Pickleball, a popular game of modified tennis, our team created PlayFit. PlayFit is an exercise program in which adults come together three times each week to play a range of sports, all modified to 1) reduce effort (e.g., smaller playing area), 2) reduce injuries (e.g., lightweight low-pressure balls) and 3) reduce competitiveness (e.g., teams chosen randomly, minimize physical contact, no keeping score). Pilot work in the summer of 2016 with 22 adults identified five modified sports that all had MET levels between 5.5 and 6.5 and high satisfaction scores (average of 8.0 out of 10). More importantly, the comments suggest that we have created a social, enjoyable experience: “that was SO much fun”, “I’ve never had so much fun playing soccer”. People liked “the laughter during the games”, “no pressure, not competitive” and that “it’s not about ability, it’s just about having fun with people”. In this study, we propose testing whether PlayFit leads to greater fitness gains, compared to group fitness, to help providers understand in a way that helps providers and fitness center staff understand: “What exercise program should I recommend to promote long-term adherence and fitness?”. All subjects will be expected to exercise three-times weekly for 60 minutes for 12 months. We will randomly assign 280 adults to two conditions: 1) Group Fitness activities and 2) PlayFit. We hypothesize that both PlayFit and Group Fitness will increase V02max significantly at 6 months but, after 12 months, mediated by greater enjoyment and higher adherence, participants randomized to PlayFit will have greater fitness and activity gains at 12 months than Group Fitness. The results of this study have the potential to help patients, providers and fitness center directors by answering the question: “What exercise program should be recommended to sedentary adults in order to promote long-term adherence and fitness?"