There have been few studies on the long-term performance of university spin-offs (USOs). This paper builds on previous empirical research into the performance of USOs and on the resource-based model of USOs. Several research issues are addressed including, in particular, the long-term performance of Swedish USOs, the distribution of the main types of business, the extent to which USOs have been in contact with support organizations, and by how much the potential growth of USOs has been achieved by the dissemination of technological knowledge to other businesses through patents, licences and other activities of the founder of the USO. The results of studies of 25 Swedish USOs, all started between 1962 and 1990 and with a median age of 25 years, confirm previous research on USOs which experienced limited growth. The analysis indicates that most of the USOs usually operate as ‘simple’ types of business, such as consultancy services; and that 60% have been in contact with support organizations.