This study aimed to investigate the effects of a mobile phone app linked to hospital electronic medical record (EMR) system on weight reduction and obstructive sleep apnea (OSA). Adults aged > 20 years with witnessed snoring or sleep apnea from a sleep clinic of a tertiary center were prospectively enrolled, and the participats were randomized into app user and control groups. The mobile app was designed to collect daily lifestyle data by wearing a wrist activity tracker and reporting dietary intake. At the visit after 2 weeks of use, a summary of the lifestyle was displayed on the hospital EMR and reviewed by both physicians and app users. Further lifestyle modification was encouraged on the basis of the electronically collected data. In the control group, the lifestyle modification performed as the usual practice. All participants underwent Watch PAT and body mass index (BMI) measurement twice: on enrollment; and after 4-weeks follow up. Changes of BMI, and parameters of sleep related breathing were analyzed.