Patients with juvenile idiopathic arthritis (JIA) have been shown to have elevated levels of circulating immune complexes (CICs) which correlated with disease activity. Our aim was to assess B cell activity by measuring the amount of and the κ:λ chain immunoglobulin light (L) chain ratio in CICs from JIA patients and to determine potential evidence for either an antigen-driven response or B-cell receptor editing. We used an enzyme-linked immunosorbent assay to measure κ and λ chains present in the CICs from the sera of patients with JIA. Statistical analysis was performed using Pearson's correlation, one-way ANOVA and Bonferroni post hoc analysis. Sera from 44 JIA patients were examined for the concentration of L chains in CICs. Healthy controls had a κ:λ chain ratio of 1.2:1, whereas this ratio was reversed among JIA subgroups with RF-positive polyarthritis (1:1.2), RF-negative polyarthritis (1:1.3), oligoarthritis (1:2.3) and systemic-onset arthritis (1:2.5). In addition, overall λ chain selection was not significantly associated with a particular immunoglobulin heavy (H) chain and occurred with all immunoglobulin isotypes. We showed preferential selection of λ chains contributing to the formation of potentially pathogenic CICs from JIA patients, of all onset types compared to healthy controls, in an H chain-independent manner. The reversal of κ:λ chain ratio within the JIA CICs and association with all immunoglobulin isotypes demonstrated the potential for L chain editing. Furthermore, we conclude that a reversal of the normal κ:λ chain ratio in JIA CICs may be used as a marker for increased B-cell activity.