Adult-onset Still's disease (AOSD) remains a perplexing, difficult to diagnose clinical entity, with clinical characteristics that are often broad and encountered in numerous other clinical entities. This vague clinical presentation is depicted in the commonly used diagnostic criteria, as the ones by Yamaguchi and Fautrel. The authors sought to investigate how diagnostic criteria apply in a series of 22 new cases of AOSD patients presenting with fever of unknown origin (FUO) and diagnosed at the Internal Medicine Department of Hatzikosta General Hospital of Ioannina, Greece. The aims of the study were: (1) to study the incidence of AOSD and (2) to retrospectively apply different classifications to the data of these patients in search of a more efficient way of diagnosing these patients in the future. The annual incidence of AOSD was estimated at two new cases per 10(5). The clinical manifestations of the patients are discussed, with an emphasis on specific manifestations being considered as criteria by Yamaguchi and Fautrel classifications. Four patients exhibited markedly increased serum D: -dimers, a finding of which the potential pathophysiologic implications are discussed. Serum ferritin levels have additive values, both for diagnostic and cost-reduction purposes in cases presenting as FUO; serum ferritin values are not included in any diagnostic set of criteria at present. The finding of high levels of D-dimers in AOSD needs further studies.