Reason for performing study: Cheek teeth (CT) diastemata are a significant cause of painful dental disease in horses and limited factual information is available on the treatment of this disorder. Objective: To assess the response to mechanical widening in the treatment of diastema. Method: Details of the presenting signs and treatment of cases of CT diastema by widening were recorded and the response to treatment of these cases assessed by re-examinations and by obtaining follow-up information from owners. Results: Sixty horses of various breeds, median age 9 years, with clinically significant CT diastemata were classified as having primary (inadequate rostro-caudal CT compression: n = 29) and secondary (to other dental abnormalities: n = 31) diastemata. Quidding was present in 82% of cases, weight loss in 35%, halitosis in 17% and bitting disorders in 12%; 85% of the identified CT diastemata involved mandibular CT and 15% maxillary CT, with caudal CT more commonly affected. Of 273 identified diastemata in the 60 horses, 207 were deemed to have significantly associated periodontal food pocketing and were widened to 4.5-5.5 mm diameter on their occlusal surface using a mechanical burr. The 29 horses with primary diastemata received a median of 1.3 treatments each and at 10 months following their last treatment 72% showed complete remission of signs and 28% showed good improvement. The 31 horses with secondary diastemata received a median of 2.2 treatments each and, at mean 9 months following their last treatment, 48% showed full improvement, 42% good improvement, 6% slight improvement and 3% no improvement. Inappropriate areas of the clinical crown were removed from 4 CT without causing pulpar exposure; one case was reported to have a transient oral infection post treatment and another developed an apical infection in a treated tooth that was unrelated to the diastema widening. Conclusion: Diastema widening is an effective treatment of periodontal pocketing in CT diastemata Potential relevance: The described treatment of CT diastema widening by trained personnel is an effective and safe treatment, but repeated treatments are often necessary, especially with secondary diastemata.