Businesses often benefit by forming alliances with other firms but risk becoming dependent on their partners. We discuss two situations in which dependence may create serious problems: first. if a partner shuts down and. second. if a partner forms a relationship with a new partner. We examine collaborative relationships fo rmed by businesses operating in the U.S. hospital software systems industry during the 1961-91 period. We find that businesses faced increased risk of dissolution if they did not form new partnerships after partners shut down or formed collaborative relationships with new pa rtners. The results have implications for del-eloping an evolutionary theory of business strategy and performance. Our approach implies that the performance of a focal business often depends 011 how the strategies of its business partners evolve over time. An evolutionary theory of strategy must incorporate key characteristics of actions and relationships throughout a web of business partnerships. The dual nature of interfirm relationships. which both help a business survive at one time and inhibit its ability to adapt at another. helps explain why so many successful businesses fail when their environments change. Formal interfirm collaboration has become an important means by wh ich busine sses in many industries gain access to capabilities needed to compete in changing markets. Empirical research suggests that collaborating firms sometimes realize corporate financial benefits (Berg, Duncan,