The inflammatory cytokines interleukin 1β (IL-1β) interleukin 6 (IL-6), tumour necrosis factor alpha (TNFα) and the anti-inflammatory peptide—the interleukin 1 (IL-1) receptor antagonist—were measured in the plasma of children with juvenile chronic arthritis (JCA). In the two subgroups studied (polyarticular JCA and systemic JCA), there was good correlation between laboratory measures of disease activity C-reactive protein (CRP), erythrocyte sedimentation rate and clinical scores for disease activity. Despite higher levels of CRP in the systemic group IL-1β levels were lower and regression analysis recorded a difference in the relationship between CRP and IL-1β within thetwo clinical groups. In contrast, IL-6 levels were high in the systemic group and correlated with disease activity. No such correlation was observed in the polyarticular group. Five children with systemic JCA were studied during the febrile phase of their illness. IL-6 levels rose and fell with the fever. TNFα levels also rose and fell but out of phase with the fever. In contrast IL-1β levels were either undetectable throughout the febrile episode or only became detectable as the temperature reduced to normal. The IL-1 receptor antagonist was usually found in 1000-fold excess over IL-lβ, levels rising and falling with the fever. These results demonstrate difference in the cytokine profiles and acute phase protein responses in polyarticular and systemic JCA. This would suggest different pathogenic mechanisms for these two groups of JCA with IL-6 being the more important cytokine in systemic JCA.