Object-oriented systems development has attracted great interest in the information systems (IS) field because of a belief that using object-oriented development (OD) makes it easier to develop and maintain software plus achieve software reuse. However, the transition from structured to OD may be especially challenging for IS developers. In fact, some argue that structured development (SD) knowledge may interfere with the ability to learn OD approaches because of a form of proactive interference. To understand how knowledge of SD concepts influences the ability to gain OD knowledge, we assessed a team of IS developers' understanding of structured and object-oriented concepts at the beginning of their first OD project and repeated our assessment 1 year later. Developers increased their understanding of some object-oriented concepts, but these changes were associated with increases in understanding of one of the SD concepts. Further, at the beginning and end of the project, developers' mental models were highly consistent with one another and that of an OD expert with regard to most OD concepts. It appears that the formal training that developers received allowed them to attain a good understanding of most object-oriented concepts. These findings challenge the idea that structured developers must ‘unlearn’ SD to adopt OD approaches. Future research may wish to examine the different types of software development knowledge to ascertain which are amenable to positive transfer and which are more likely to suffer from proactive interference.