Treatment of diabetic patients strongly relies on the continuous logging of parameters relevant to glycemic control. Keeping diabetes diaries can be tedious which can affect the data quality and completeness. Mobile technologies could provide means to overcome these limitations. However, studies analyzing the direct effect on the treatment of patients are rare. In the presented study diabetic patients were supplied with a smartphone application to record various parameters relevant for glycemic control. Questions regarding the completeness of diabetes diaries were answered by the patients before and after the study. The attending diabetologist analyzed the data obtained from the smartphone-based diaries to determine whether these provided solutions for problems in glycemic control. The analysis of the available smartphone data provided the basis for therapeutic recommendations that can improve the daily glycemic control for almost all participants. Importantly, especially the newly developed implicit-activity logging, registering the participants' movements, provided important means to generate these recommendations.