Concepts taken from complex systems theory, such as ‘agents’ and ‘attractors’, have been proposed as metaphors in medical practice. This proposal is assessed by a comparison of the notions of complex adaptive systems (CAS) and human complexity. CAS are characterized by the emergence of sophisticated output features of rule-governed non-linear systems. Human complexity is the result of higher mental capacities and human culture. Failure to understand it in such terms may result in medical failure or medical ‘success’ in which the wrong problem is solved. CAS concepts may thus be useful in the solution of given medical problems, while their application may lead to an undesirable reduction of complexity in cases in which the identifi cation of the medical problem itself is part of the medical challenge.