Regression analysis in MPS I revealed that the within‐individual change in IL‐6 over the first 2 years of the study was significantly associated with the change in joint ROM over years 5 to 9 (−22°/year change in ROM for every 1 pg/mL increase in IL‐6 (95%CI −29 to −15; P < .001); this association remained significant when repeating the analysis with only MPS IH participants (P < .001) confirming that the association is not due to differences between MPS IH and MPS IA. The gene discussed is IL6; the disease is Scheie syndrome.