During the early stages of design, the designer forms an initial understanding about the problem and some ideas on how to solve it, often influenced by previous design knowledge. In order to support HCI design in this context, we investigated ways to use case-based reasoning (CBR) to help designers access and reuse design knowledge to solve new HCI design problems. We conducted interviews with professional HCI designers to collect data about how they deal with design problems, and their motivations and expectations regarding the use of design knowledge aided by a CBR tool. Using this data, we designed and developed a tool called CHIDeK, which has a library containing HCI design cases and provides access to them through faceted navigation, direct links between cases, and search. To investigate the way CHIDeK influences the design activity, we conducted a study that simulated the early stage of HCI design of an online bike reservation system. Some participants could solve the problem while having access to CHIDeK and others had to solve it without CHIDeK. We discovered that the cases from CHIDeK supported the design by motivating the designers reflective process, triggering their memories of experiences with systems similar to the ones in cases, and helping generate new ideas. We also identified some limitations in the case representation, which offers an opportunity for further research. When comparing both kinds of design activities, we noticed that designers without the case library used the same solution for one of the issues described in the study scenario, while the designers with the cases varied between two solutions. We concluded that a CBR tool has much potential to aid the design activity, but there are still issues that need to be addressed by further research.