The demonstration of low arylsulfatase-A (ASA) activity in leucocytes or fibroblasts is used often to establish the diagnosis of metachromatic leucodystrophy (MLD). However, low ASA activity is observed also in pseudo-ASA deficiency which may be as common as MLD. We report two patients with pseudo ASA deficiency who had abnormal neurological findings consistent with atypical MLD. Because the measurement of ASA activity is neither a sensitive nor specific method with which to establish a diagnosis of MLD, this diagnosis should be confirmed by nerve biopsy, measurement of urinary sulfatide or a cerebroside sulfate loading test, using cultured fibroblasts.