Inversion injuries of the ankle are common and most are managed adequately by functional treatment. A significant number will, however, remain symptomatic. Synovial impingement is one cause of continuing pain. This condition is often difficult to diagnose because the physical signs and investigations are non-specific. If the diagnosis is made, treatment by arthroscopic debridement has been shown to be highly effective. Our aim was to describe a new physical sign to help in the diagnosis of anterolateral synovial impingement in the ankle. A cadaver dissection demonstrated the anatomical basis for the physical sign and a prospective clinical study involving 73 patients showed that the lateral synovial impingement test had a sensitivity of 94.8% and a specificity of 88%. We describe the test and conclude that this physical sign will be of use to practitioners treating patients with chronic pain in the ankle after injury.