The aim of this study was to describe the effect of sex, growth, Tanner stage, and physical activity on knee articular cartilage volume development. A total of 74 randomly selected male and female children aged 9–18 y were measured on two occasions at an average interval of 1.6 y (range 1.3–1.9). Articular cartilage volume was determined at the patella, medial tibial, and lateral tibial compartments by processing images acquired in the sagittal plane using T1-weighted fat saturation magnetic resonance. Height, weight, and BMI were measured while Tanner stage and physical activity were assessed by questionnaire. Articular cartilage volume increased at all sites peaking in Tanner stage two. Males gained articular cartilage faster than females at all sites (patella+ 233 μL/y, 95% CI− 7,+ 473, medial tibial+ 350 μL/y, 95% CI+ 118,+ 582, lateral tibial+ 256 μL/y, 95% CI+ 22,+ 488). In both sexes, articular cartilage volume accrual at tibial but not patella sites correlated significantly with height change but not weight change. Overweight children did not differ significantly from normal children in articular cartilage volume either cross-sectionally or longitudinally. The most consistent physical activity association was with average intensity of sport with those above the median gaining approximately twice as much as those below the median at tibial (p< 0.05) but not patella sites. In conclusion, most children gain articular cartilage during growth, but there is wide variation in the amount of articular cartilage accrual. In particular, younger children, males, and those undertaking more vigorous sports have substantially higher accrual rates. These results provide novel data on articular cartilage development in humans. The long-term significance of these results with regard to osteoarthritis of the knee in later life remains hypothetical.