This paper presents a hybrid case-based reasoning system for on-line technical support of PC fault diagnosis. HyCase consists of a natural language (keyword) input and the graph-theoretic constraint-net.Natural language or keyword inputs are parsed and then generated into a constraint-net. The constraint-net is validated and its links rationalized and standardized to minimize ambiguity. Cases that partially match either the keywords or the constraint-net are ranked according to their matching scores based on four different preferences, and then retrieved and presented to the user. The case base, which was developed in Microsoft Access 2000, is updated by means of a keyword and constraint-net manager. HyCase was implemented on a PC with a Pentium III processor running at 500 MHz and 128 MB of SDRAM.Twenty typical queries from customers were tested on a collection of 174 cases based on different cut-off overall matching scores. The effectiveness of case retrieval was measured by the proportion of relevant cases retrieved from the case base (recall) and, of these, the proportion directly applicable to the problem at hand (precision). The accuracy of the natural language parser was ascertained to range between 62.5% and 87.7%, while a parsing accuracy of 60% is sufficient to ensure a reasonable recall and precision. A minimum overall matching score of about 0.5 ensures a precision of 0.60. The parsing time gets noticeably longer when there are more than 15 words. Except for minimum overall matching scores exceeding 0.1, the parsing time is largely independent of the minimum score.The merits and limitations of HyCase are also discussed.