Background: Injury resulting in proliferative new bone formation on the dorsal aspect of the middle phalanx (P2) has been recognised by equine veterinarians and is generally referred to as capsulitis. Objectives: Describe the clinical features, treatment, and outcomes of this condition. Study Design: Case series (n=19) Methods: Clinical cases in which the horse presented for lameness and were diagnosed with acute formation of proliferative bone on the dorsal aspect of P2 were included in the study (cases collected from 2005-2022). Signalment, history, presenting clinical signs, diagnostics, treatments administered, and outcome were evaluated. Results: Horses presented with unilateral lameness that were all classified as either a moderate lameness at a trot (n=6), or a moderate to severe lameness at a walk (n=13). The presentation of lameness was recorded as acute in 15/19 horses and not reported in the remainder. Bone proliferation on the dorsal aspect of P2 was diagnosed on radiographic evaluation in all horses (as part of the inclusion criteria). In horses with sequential radiographs made from the time of acute presentation, the bone production developed rapidly over a period of a few weeks to 2 months. A variety of therapeutic options were implemented, and 13/19 (68%) of the cases were subject to humane euthanasia with only one horse returning to its previous athletic function. Milder cases were more likely to have better outcomes. Main Limitations: Small sample size. Conclusions: Acute new bone proliferation on the dorsal aspect of P2 may be associated with moderate to severe lameness and carries a poor prognosis for return to athletic performance and survival.