It has l)een remarked tliat the brains of most children with mongoHsm wrigh less than normal, though none shows extreme reduction in size, and that the smallness of the brain-stem and cerebellum is a special feature (Grome in Hilliard and Kirman, 1965). We have made a statistical comparison of values relating to mongols and to normal subjects with respect to the weights of the whole brain and the cerebellum and brain-stem. METHOD AND RESULTS1 he total brain weight and the weight of ttir brain-.stem and cerebellum were obtained from the post-mortem examinations of 19 patients with mongolism in the Fountain Group of Hospitals {Table 1). These were compared, matching for age and sex, with the values reported by Krogman (1941) for Crerman children from birth to 14 years, taking these as a standard for normal sul)jects.The ratio of observed to expected weights showed both a diminution of the total brain weiglit and a di.sproportionately more extreme diminution of the weight of brain-stem and cerebellum, taken together. Whereas tbe brain as a whole was reduced on average to 76% of its normal weight, the weight of brain-stem and cerebellum was on average less tban f>6% of normal. Tins difference of 10% is highly significant statistically. Within individuals the two weights are correlated with one another; in 17 out of the 19 ca.ses the ratio of observation to expectation was higher for total brain than for brain-stem and cerebellum, and in only two cases {Nos. 11 and 17) was the opposite true. The mean difference between these two ratios witliin individuals was 10.2% ± 2.3%, for which t^4.43, p<.()01 {see Table 2). It appears, therefijre, that a disproportionate reduction in the size of the cerebellum and brain-stem may be a specific feature of mongulisni.Another way of looking at the data is not without interest. Reference to Krogman's tables shows that, in tlif American standards he quotes, the proportion of the total brain weight made up by the combined weight of cerebellum and brainstem is very constantly 12% after the age of 1 year. At birth the proportion is 7.3%, at 3 months 7.9%, at 9 months 11.0% and at 1 year 11.5%. From the data .supplied about Cierman children, we can calculate the proportion as 12.48% in boys and 12.97% in girls, these two figures not differing significantly from one another, and both ))eing invariant in respect of age from one year onwards. The mean fur both sexes is 12.7%, and the standard deviation 1.1%. Table I shows the percentage in each of our 19 cases. It will he s;een that in all but two cases the value is less than (Received February 24th, 1966)

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“The study of phenylpyruvic amentia may throw light on the whole problem of mental deficiency.”—Jervis.Phenylpyruvic oligophrenia is a syndrome in which mental deficiency is accompanied by the excretion of phenylpyruvic acid in the urine.