Background: The world over, the multimorbidity associated with aging is proving to be a great socio-economic burden. The loss of muscle mass and increased adiposity are typical of aging, but the number of longitudinal studies focusing on sarcopenia or sarcopenic obesity is currently limited. We launched a hospital-based, prospective cohort study of aged people (HAPPY cohort) to identify the impact of sarcopenia or sarcopenic obesity on the occurrence of chronic diseases including diabetes, or its vascular complications, osteoporosis and fractures, cognitive dysfunction, and mortality. Methods: For the HAPPY cohort, we aim to recruit 1,000 patients, aged 60 years and above, who visit a tertiary hospital from August 2015. Participants are scheduled to undergo repeated waves of assessment every 3 years; this will be done for 9 years or until death. Results: General information and medical histories are collected through personal interviews and validated using medical records. Participants' physical function is assessed by diverse functional assessments including the hand grip strength test, timed up and go test, and the chair rise test. The functional status, nutritional status, levels of depression, and cognitive function of the participants are also measured. The body compositions are assessed using dual energy X-ray absorptiometry and the bioelectrical impedance method. The biochemical parameters related with chronic diseases are measured. Conclusion: This HAPPY cohort study will provide results relevant to the many health issues among elderly Asians (among at least elderly Koreans). These results will help us gain a better understanding of sarcopenia and sarcopenic obesity, as well as the cross-sectional and longitudinal association of chronic diseases and mortality among elderly adults.