Two cases of adult metachromatic leucodystrophy in one family are reported. The main clinical features in both were predominantly psychiatric with alcoholism and an extremely long duration of the illness. Neuropathological examination revealed a similar distribution of the lesions in both, and scanty metachromatic accumulation in the CNS and not at all elsewhere. The great variability of the lengths of the courses is stressed. The duration in no way seems to be linked to the age of onset, except in the typical infantile form. The authors argue that different lengths of history correlate with distinct neuropathological findings, and may possibly be related to qualitative differences in the involved enzyme disturbance. Therefore, the authors suggest that the classification based on the age of onset be enlarged with a further one distinguishing between 'rapid' and 'slow' course types.