The most important trigger factor for attacks was exercise, which was present in 27 (96%) of our 28 patients, similar to the series of DiMauro and Papadimitriou.8 Attacks were not triggered by exercise in only 1 patient. Other trigger factors were infections in 13 patients (46%), fasting or low nutritional intake in 5 (18%), and cold in 4 (14%). In 1 patient, an attack was triggered by emotional stress. Often attacks were induced by a combination of trigger factors (eg, extensive skiing in the cold without appropriate food intake). Moreover, there are reports that attacks can be triggered by drugs (eg, ibuprofen,13 very high doses of diazepam,14 and valproate sodium15) or by general anesthesia.16 Frequencies of signs and trigger factors are given in Table 1, and detailed clinical features of our patients are given in Table 2. Remarkably, infections were a more frequent trigger factor than fasting in our patients compared with the series of DiMauro and Papadimitriou.8 Severity of exercise that triggered symptoms and frequency of symptoms were highly variable. In some patients symptoms were induced only by heavy or very long-term exercise, such as mountain hiking, whereas in others symptoms were triggered by mild exercise, such as strolling. Frequency of the attacks ranged from a single attack to attacks every month. In addition to the attacks, some patients complained of moderate but frequent exercise-induced myalgia; thus, daily activities were impaired. Three patients never had severe attacks but did have frequent exercise-induced myalgia.