Interactive case simulation tools (ICSTs) are important vehicles to disseminate medical knowledge. We conducted a study to examine the usage of an insomnia screening and treatment case simulation tool in an HIV clinical education program. Using system usage diagrams (SUDs) as an instrument, we quantified visit frequency and length of stay for different types of system resources. Preliminary results have shown that both recommendations and interactive decision diagrams were frequently used, with the former having a longer length of stay but fewer visits. Case simulation functions seemed to be able to engage users. Future research is required to verify the generalizability of the identified usage patterns, to investigate issues in usability design, and to perform correlation analyses on system usage and context parameters.