The authors examined the thinking processes in the use of a decision support system (DSS) by social workers in a human services agency to determine whether they used the system to improve their case reasoning. Information was obtained from in-depth interviews with eight social workers who used a DSS in their work and from content analysis of "narrative justifications" appended to 1,074 decisions that differed from those recommended by the DSS. Findings show that the social workers used the DSS in a perfunctory manner in typical cases, but as an aid to thinking and reflection in atypical cases, in which they were uncertain of how to decide. On the whole, the social workers showed little interest in the DSS's recommendations, but reported that the process of entering data and answering the model's questions about the case were useful when the case was atypical. The authors suggest that computer support systems should be designed to help workers think through atypical cases.