Given that the design activity makes use of previous design knowledge, we turned to case-based reasoning (CBR) to help identify opportunities to support the design of human-computer interaction (HCI). Using interviews with professional designers and Semiotic Engineering, we developed a CBR tool called CHIDeK (Computer-Human Interaction Design Knowledge), with which we conducted a study to observe how it influenced the HCI design activity. We found that the cases recorded in CHIDeK supported design by motivating the designers’ reflective process, triggering their memories of experiences with similar systems, and helping to generate new ideas. We have also identified limitations in our case representation and case access methods, which offer opportunities for further research.