In order to analyse possible triggering or contributing infections and HLA-B27 frequency in patients with acute febrile joint syndrome fulfilling the proposed criteria of adult Still's disease (AOSD), we studied prospectively the serological findings of 25 patients. They were aged 15-62 years and diagnosed between 1978-1992. We then compared results with a control group consisting of 119 healthy persons. Positive viral or bacterial serology was found in 12 patients (48%) in the AOSD group compared with 13 cases (11%) in the control group (p < 0.001). Fourfold or higher viral antibody rise was found in two patients and bacterial antibody rise in three patients. High stable viral antibody titre was observed in one patient and high stable bacterial antibody titre in six patients. HLA-B27 was not overrepresented in the study group (12%) compared with a healthy Finnish population (14%). We conclude that many different bacterial and viral infections may trigger or contribute to AOSD.