Sensitivity and specificity of antiperinuclear factor (APF) determination were assessed in 127 patients with classical or definite rheumatoid arthritis (RA). The control population consisted of 262 patients with rheumatic complaints other than RA. Our results showed a sensitivity of 86.6%, a specificity of 96.2% and a Youden index of 82.8% for APF in RA. In 14.2% of the total population with RA the rheumatoid factor (RF) became negative in the course of the disease; all but one were found to be actually APF positive. Of the population with RA analyzed, 8.6% were consistently RF negative but APF positive. In view of its strong sensitivity and specificity for RA, APF determination is a valuable serological tool for the diagnosis of the disease. In APF positive, RF negative patients with RA the prognosis seems to be poor.