Telemedicine applications have the potential to enhance patient's safety at home by remote monitoring of chronic diseases. Telemedicine involves the interaction between multiple user groups through a system, making the usability aspect of such system crucial for the continuous, efficient and satisfactory use of the application. The main objective of this study was to carry out a usability evaluation in the field of a telemedicine application for remote monitoring of chronic obstructive pulmonary disease (COPD) patients to improve the application's user interface before system deployment. A field trial was performed with six COPD patients at their homes, continuously using the system's application on a tablet for seven days. The usability evaluation identified 23 usability problems related to users' interactions and system's functionality. These problems were solved with the refinement of the system through an iterative application development process. The outcome of the study was the improved telemedicine application that was adopted by the partners of the FP7 EU project United4Health.