Background: Ongoing arthritis in children with juvenile idiopathic arthritis (JIA) can result in cartilage damage. Objective: To study the feasibility and repeatability of T 1ρ for assessing knee cartilage in JIA and also to describe T 1ρ values and study correlation between T 1ρ and conventional MRI scores for disease activity. Materials and methods: Thirteen children with JIA or suspected JIA underwent 3-tesla (T) knee MRI that included conventional sequences and a T 1ρ sequence. Segmentation of knee cartilage was carried out on T 1ρ images. We used intraclass correlation coefficient to study the repeatability of segmentation in a subset of five children. We used the juvenile arthritis MRI scoring system to discriminate inflamed from non-inflamed knees. The Mann-Whitney U and Spearman correlation compared T 1ρ between children with and without arthritis on MRI and correlated T 1ρ with the juvenile arthritis MRI score. Results: All children successfully completed the MRI examination. No images were excluded because of poor quality. Repeatability of T 1ρ measurement had an intraclass correlation coefficient (ICC) of 0.99 (P<0.001). We observed no structural cartilage damage and found no differences in T 1ρ between children with (n=7) and without (n=6) inflamed knees (37.8 ms vs. 31.7 ms, P=0.20). However, we observed a moderate correlation between T 1ρ values and the juvenile arthritis MRI synovitis score (r=0.59, P=0.04). Conclusion: This pilot study suggests that T 1ρ is a feasible and repeatable quantitative imaging technique in children. T 1ρ values were associated with the juvenile arthritis MRI synovitis score.