Smartphone applications (apps) have been used and evaluated in the context of workplace health promotion (WHP) programs. However, there is a lack of studies analyzing actual app usage data and measuring changes in medical markers to evaluate the effectiveness of WHP apps in terms of health improvements in practice. In this study, we evaluated data from 555 employees of an IT company who participated in a WHP program over the course of one year. Participants of the program received a medical check-up as well as a health app to understand their medical results and receive advice for a healthier lifestyle. In addition, 99 of these employees underwent a follow-up medical check-up. It was found that the smartphone app for healthy lifestyle promotion in combination with onsite medical check-ups was effective in improving various health indicators, for example, BMI, body fat, blood pressure and triglycerides. The study further identified influence factors for sustained app usage, and analyzed different usage behaviors among gender and age groups.