SummaryReason for performing study: Cheek teeth diastemata are a common cause of painful periodontal disease in horses, but there is limited objective information on their treatment. Objective: To assess the long-term response to diastema widening in clinically affected horses. Study design: Retrospective study. Method: Medical records from cases of cheek teeth diastemata treated by diastema widening referred to the University of Edinburgh Equine Hospital from 2008 to 2011 were analysed. Results: During this period, 302 horses were diagnosed with clinically significant cheek teeth diastemata, of which 202, median age 11 years, with severe associated periodontitis were treated by widening of 674 problematic diastemata; 89.8% between mandibular cheek teeth and 10.2% between maxillary cheek teeth, with a mean of 1.5 treatments performed per case. These 202 cases showed quidding in 76.2%; weight loss in 33.2%; bitting problems in 20.1% and halitosis in 10.9%, with 5.4% being asymptomatic. Follow-up of 92% of treated cases, a mean of 20.8 months after their initial treatment, showed that 72.6% had complete remission of clinical signs that was permanent (for the duration of this study) in 50.5% and temporary in 22%. A partial response was obtained in 17.2%, no response was obtained in 4.3%, and owners were unsure of response in 5.9%. Clinical improvement was sometimes delayed, with 19% taking >4 weeks following treatment for improvement. Inappropriate sites were burred in individual teeth of 6 horses, causing iatrogenic pulpar exposure in 2 cases, but following treatment none developed clinical signs of apical infection. Conclusions: Diastema widening is an effective but potentially invasive treatment for horses with cheek teeth diastemata with severe periodontitis. Potential relevance: Diastema widening by trained personnel is suitable for advanced cases of cheek teeth diastema, but many cases require repeated treatments.