A previously asymptomatic 30 year old man presented with rhabdomyolysis, muscle weakness, and acute encephalopathy after strenuous exertion in the cold without adequate food intake. Serum and muscle carnitine concentrations were decreased. Urinary excretion of carnitine and glycine esters and biochemical examination of skeletal muscle and fibroblasts led to the diagnosis of medium chain acyl-CoA dehydrogenase (MCAD) deficiency. A point mutation at nucleotide position 985 of the coding region of the MCAD gene was found. The MCAD protein was synthesised in the patient's fibroblasts at a normal rate, but was unstable. In general, patients in whom the 985 point mutation has been established show much more severe clinical symptoms and other symptoms than those seen in this patient. The relation of the 985 point mutation and the residual MACD activity to the symptoms is not as straightforward as previously thought.