OBJECTIVE: To investigate the influence of arthritis in individual joint groups on subdimensions of functional ability questionnaires in children with juvenile idiopathic arthritis (JIA). METHODS: 206 patients were included who had the Childhood Health Assessment Questionnaire (C-HAQ) and the Juvenile Arthritis Functionality Scale (JAFS) completed simultaneously by a parent and received a detailed joint assessment. In each patient, joint involvement (defined as presence of swelling, pain on motion/tenderness and/or restricted motion) was classified in 3 topographic patterns: Pattern 1 (hip, knee, ankle, subtalar and foot joints); Pattern 2 (wrist and hand joints); Pattern 3 (elbow, shoulder, cervical spine and temporomandibular joints). Frequency of reported disability in each instrument subdimension was evaluated for each joint pattern, present either isolatedly or in mixed form. RESULTS: Among patients with Pattern 1, the JAFS revealed the greatest ability to capture and discriminate functional limitation, whereas impairment in the C-HAQ was more diluted across several subdimensions. Both C-HAQ and JAFS appeared to be less reliable in detecting functional impairment in the hand and wrist (Pattern 2) than in other body areas. Overall, the JAFS revealed a superior ability to discriminate the relative functional impact of impairment in individual joint groups among patients with mixed joint patterns. CONCLUSION: In children with JIA, a functional measure focused to assess the function of individual joint groups (the JAFS) may detect with greater precision the functional impact of arthritis in specific body areas than does a standard questionnaire based on the assessment of activities of daily living (the C-HAQ).