Hirschheim, Klein and Lyytinen introduced the four-paradigm theory of information systems development (ISD) as a significant attempt to systematise developer assumptions. The theory perspective is that developers hold key assumptions that may be grouped together and classified into paradigms, and that these paradigms influence their ISD behaviour. The aims of the research described here are theory exploration and explanation in case studies concerning the ISD process in three public National Health Service (NHS) institutions in the north of England. We focus on the behavioral rather than the cognitive (assumptions) aspect of the theory. Our conclusions are, firstly, that qualitative theory explanation is desirable because we need to test theory in practice to show its applicability to wider settings. A rigorous qualitative, interpretive method, paying attention to openness and validity, can satisfactorily undertake such theory explanation; such research can help our IS community to gain wider credibility, authority and acceptance. Secondly, with regard to the four-paradigm theory, its predictions were largely met, as the paradigms were capable of classifying developer behaviour and developers had a dominant paradigm, namely functionalism. We found the theory to be very relevant to the investigation of current IS issues, and we introduce the concept of developer paradigmatic inconsistency.