Changeover from the traditional paper-based patient records to the computerized patient records has been spreading rapidly in medical facilities. Since ordinary input devices such as a keyboard and mouse are often thought insufficient for the needs of medical workers using electronic patient records (EPR) compared with traditional pens and paper-based patient records, a pen-tablet system (PTS) as a digital pen interface tends to be used instead. On the other hand, with the growing adoption of thin-client computing (TCC) in medical facilities, the usability of PTS with TCC has come to be a concern because of possible reduction of the usability due to delays of response time occurring in TCC environments. To analyze the factors that influence the usability of PTS with TCC, the authors focused on the relationship between length and its scattering of the response time delay. The results indicated that the delay scattering could be a more influencing factor than the delay length itself and that values of the scattering should be within approximately 35 milliseconds for the best usability. This study would provide useful indicators for evaluating the usability of PTS in incorporating it into the EPR system with TCC environments.