This study explores the diagnostic process of general practitioners confronted with ill-defined and ambiguous complaints, which eventually appeared to be caused by a malignancy. Three aspects were rated: (a) the adequacy of the initial problem definition; (b) the carefulness of further diagnostic methods; and (c) how the suspicion of malignancy originated. These three aspects, which were strongly connected, seem to be parts of a diagnostic approach with two polar extrems: a critical style and a biased style. Characteristic of a critical style is full awareness of detail, careful observations, consideration of ambiguous symptoms, and consciousness that the correct diagnosis is often other than the one initially judged most likely. The opposite, the biased style, is characterized by little alertness for detail, less careful observations, and overinterpretation of facts supporting the initial hypotheses.