Workplace wellness programs cover over 50 million workers and are intended toreduce medical spending, increase productivity, and improve well-being. Yet, limitedevidence exists to support these claims. We designed and implemented a comprehensiveworkplace wellness program for a large employer with over 12,000 employees, and ran-domly assigned program eligibility and financial incentives at the individual level. Over56 percent of eligible (treatment group) employees participated in the program. We findstrong patterns of selection: during the year prior to the intervention, program partic-ipants had lower medical expenditures and healthier behaviors than non-participants.However, we do not find significant causal effects of treatment on total medical expendi-tures, health behaviors, employee productivity, or self-reported health status in the firstyear. Our 95% confidence intervals rule out 78 percent of previous estimates on medicalspending and absenteeism. Our selection results suggest these programs may act as ascreening mechanism: even in the absence of any direct savings, differential recruitmentor retention of lower-cost participants could result in net savings for employers.