SummaryA 30‐year‐old woman with clinical features and biochemical findings of muscle phosphofructokinase deficiency was found to have a very low level of α‐1,4‐glucan:α‐1,4‐glucan‐6‐transglucosylase (branching enzyme, EC 2.4.1.18) activity in muscle. In contrast, branching enzyme activity in the leukocytes was in the range of control values. After sedimentation of the glycogen from muscle homogenates by centrifugation at 105 000 g, branching enzyme activity in muscle of the patient was similar to that of control subjects. This patient illustrates the possibility of falsely diagnosing branching enzyme deficiency when muscle glycogen content is elevated. It is likely that such an artefact may also cause a false positive diagnosis of branching enzyme deficiency in other metabolic diseases associated with glycogen accumulation.