The presence of comorbidity may distort research findings but, if properly dealt with, it also provides important research opportunities to test hypotheses about causal mechanisms. Seven possible sources of artefactual comorbidity are noted. Five key alternative bases for apparent comorbidity are discussed, using medical examples to illustrate the different processes that they may represent. They are that the apparent comorbidity: (1) represents two manifestations of the same disorder; (2) reflects two stages of the same underlying condition; (3) arises from the same or correlated risk factors; (4) represents a nosologically distinct condition; and (5) is due to one condition predisposing to the other. All possibilities, however, have to start with demonstration of the distinctiveness of the two syndromes involved in the comorbidity. Such evidence may be provided by differences in: (1) pathophysiology or pathopsychology; (2) correlates likely to reflect aspects of causal processes; or (3) treatment responses and adult outcome. Copyright © 1997 Whurr Publishers Ltd.