Background : The emergent nature of atypical myopathy or atypical myoglobinuria (AM) necessitates precise description of its clinical and epidemiologic features. Purpose : To define key features of AM to help practitioners recognize the disease and to advise owners to take preventive measures. Animals : Belgian cases of AM confirmed by histology (CC horses; n = 57) from autumn 2000 to spring 2005 were included in the study. Co‐grazing horses (Co‐G horses; n = 77) that remained free of any abnormal clinical signs constituted a control group. Methods : History, environmental characteristics, clinical signs, and laboratory results associated with AM were determined by a retrospective case series study. Results : Young horses in poor or normal body condition were found to be at risk for AM. Pastures were characterized by poor natural drainage and vegetation of low nutritional value. Features of AM were seasonal occurrence, apparent link with weather conditions (ie, lack of solar radiation with no heavy frost and an excess of precipitation or relative humidity), sudden onset of clinical signs, and rapid death. Evaluation of serum creatine kinase activity indicated severe muscle destruction in CC horses and subclinical disease in a few Co‐G horses. Conclusions : The association of AM with specific environmental conditions and individual animals suggests that young horses should not be pastured on bare premises subject to humidity when the weather has been very wet and cold for several days. Management of AM outbreaks should include control of Co‐G horses who are apparently healthy.