Background. Progressive pseudorheumatoid dysplasia (PPRD), a noninflammatory condition, needs to be differentiated diagnostically from juvenile rheumatoid arthritis (JRA). Objective. Demonstration of an unusually large and often early-appearing os trigonum helps distinguish PPRD from JRA. Materials and methods. Ankle images in four children with PPRD were reviewed. Results. The os trigonum was abnormally enlarged in all PPRD subjects and was shown to have appeared or fused earlier than normal in two subjects. Conclusion. A large and early os trigonum ossification helps differentiate PPRD from JRA.