Intestinal mycobacteriosis is a chronic, progressive disease which can affect horses of all ages, and has been identified in weanlings as young as six months of age. The disease causes a granulomatous enteritis/enterocolitis. Clinical signs are non-specific, and include weight loss, diarrhoea, intermittent fever and dependent edema secondary to protein losing enteropathy (PLE). Intestinal mycobacteriosis should be considered as a differential diagnosis in any horse with chronic diarrhea, weight loss and/or a protein losing enteropathy. The condition appears to be rare in Europe and in other parts of the world; however the authors have personally diagnosed seven cases of intestinal mycobacteriosis in Finland between 2009 and 2013, suggesting an apparent increase in the incidence of the disease in recent years.