Recent studies analyzed the effect of retirement on mentaland physical health. Some of them find that retirementyields a loss in cognitive skills while others find that retirementpreserves physical health. When analysing the effectof retirement against work activity, these studies do notdistinguish between part-time and full-time work activity.We study how the amount of work hours affects the physicaland mental health conditions of US residents between 50 and75 years old in eleven waves (1992-2012) of the Health andRetirement Study. To avoid the potential bias due to the factthat deteriorating health conditions can cause employeesto work fewer hours, retirement eligibility ages are used asinstruments for part-time or full-time work decisions. Wealso control for, possibly health related, unobserved heterogeneityacross individuals. We find that working part-timeor full-time deteriorates self-reported health and memory.On the other hand, part-time and full-time working reducesbody weight, and full-time working improves the word recallscore. In general, health status of the elderly responds toworking part-time much more than it responds to workingfull-time, suggesting that the effect of number of hoursworked on health outcomes is nonlinear.