This study was performed to assess the frequency of HLA B27 in patients with juvenile chronic arthritis (JCA) of varying severity and outcome by studying three patient categories: those in whom cytostatic treatment with azathioprine had been started, those with secondary amyloidosis, and those with arthroplasty of the knee or hip joints. In the first category the frequency of the HLA B27 allele was compared between those who had attained remission and those who had not. In the second and third categories the rate at which amyloidosis developed and the timing for the need of arthroplasty, were compared for HLA B27-positive and -negative patients. A control group consisted of 37 patients with uncomplicated seronegative polyarthritis. Ten of the 37 patients in the control group (27%) were HLA B27 positive as opposed to 84 out of 190 (44%) in the three study groups. Of the 101 patients treated with azathioprine, two out of 15 in remission were HLA B27 positive, whereas as many as 41 out of 86 with still active disease were HLA B27 positive (p = 0.013). Of the secondary amyloidosis patients, 29 out of 51 carried HLA B27. The HLA B27-positive patients contracted amyloidosis on average 5.9 (median 6.7) years earlier than the HLA B27-negative patients (p = 0.038). Of the arthroplasty patients, 39 out of 91 carried HLA B27. The HLA B27-positive patients underwent arthroplasty on average 2.9 (median 3.5) years earlier than the HLA B27-negative patients (p = 0.050). We conclude that HLA B27-positive cases are accumulated among the most severe cases of JCA.