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Determining how to cope with existing systems is an important issue for information systems development (ISD). In this paper, we investigate how well different ISD patterns are suited for coping with existing systems. Empirical results, gathered from three software development projects undertaken by a financial institution, suggest propositions regarding how ISD patterns and existing systems affect the characteristics of objective ISD complexity, which in turn determine overall experienced complexity. Existing systems increase complexity due to conflicting interdependencies, but ISD patterns that reduce this complexity, such as those that employ bottom-up or concurrent consideration patterns, are best suited for coping with existing systems. In contrast, top-down and iterative focusing patterns, as classically used in new development, increase the complexity associated with conflicting interdependency, which makes them particularly unsuited for coping with existing systems in ISD.