To determine the influence of breastfeeding duration in the clinical activity of low-income juvenile idiopathic arthritis (JIA). Ninety-one JIA patients followed in Fortaleza-CE, Brazil, were cross-sectionally evaluated from May 2015 to April 2016. Breastfeeding duration was obtained by interviewing mothers. Mean age was 14.6 ± 5.2 years with 10.31 ± 3.7 years of disease duration. Polyarticular category predominated, with 39 (42.8%) patients, followed by 23 (25.3%) oligoarticular and 17 (18.7%) enthesitis-related. Forty-seven (61.8%) were receiving methotrexate isolated or combined to leflunomide, which was used by 12 (15.4%); 30 (32.9%) were on biologic DMARD with 16 (53.3%) etanercept, 8 (26.7%) adalimumab, 3 (10%) tocilizumab, and 1 (3.3%) each on infliximab, abatacept, and canakinumab. Mean(SD) CHAQ and JADAS27 were 0.37 ± 0.36 and 5.03 ± 6.1, respectively and 22 (24%) had permanent joint deformities. No family declared monthly income over US$900.00 and 32 (37.2%) earned less than US$300.00. Eighty-three (91%) were ever breastfed; over two-thirds were breastfed for more than 3 months. Those breastfed for more than 6 months had less joint deformities and a tendency to lower JADAS27 and CHAQ scores using minimally adjusted general linear or logistic models, as appropriate. Parental smoking or literacy and family income did not differ regarding breastfeeding time. This is a low-income JIA cohort with the highest breastfeeding prevalence ever reported. Breastfeeding over 6 months was associated with less disease activity.Key Point• Long-term breastfeeding benefits juvenile idiopathic arthritis.