In a continuing twin study of autism in Scandinavia and Finland, moderately mentally retarded triplets fulfilling Rutter's criteria for infantile autism were reported. Judging by physical appearance the triplets were identical. Behaviourally they were extremely similar though one was intellectually slightly better than the other two. All three showed the physical stigmata characteristic of the fragile–X syndrome, in spite of their overall appearance being non-conspicuous. The triplets had between 8 and 12 per cent of fragile–X positive cells and showed a distinct pattern of urinary excretion of substances yielding absorbency at 280 nM. Their mother and sister also had a high count of fragile–X positive cells.