Essentially all cases of phenylketonuria treated with a low phenylalanine diet and published up to December, 1959, were reviewed and evaluated. All untreated individuals with phenylketonuria with an I.Q. above 60 that have been recorded were tabulated. The number of these (20) verifies as reasonable the incidence of 2.5% of 466 untreated cases of phenylketonuria that have an I.Q. above 60. This figure was used as one basis of evaluation of the treatment.

Forty-four patients with phenylketonuria, over 3 years of age, have been treated without impressive change in mental or neurologic status. These were not considered further.

Forty-three patients with phenylketonuria, treated before 3 years of age, were tabulated and evaluated in detail. The mean age at start of treatment was 16.2 months, and the mean duration of treatment was 16.8 months.

The treated group had 18 times as many with an I.Q. above 60 and twice as many with a normal electroencephalogram as the untreated group. These differences were highly significant. The treated group also had fewer seizures.

Those treated before 16 months of age, compared with those treated between 16 months and 3 years of age, had four times as many with an I.Q. above 60 and fewer (none) with seizures. These differences were highly significant. The early treated group also has more with normal electroencephalograms.

For the whole group the final I.Q. was negatively correlated with age at the start of treatment in a highly significant manner (r = -0.67, P < .01). Over the 3-year period studied, a minimal loss of nearly 5 points in the I.Q. occurred each 10 weeks that treatment was delayed.

The low phenylalanine diet started in the early months of life is therefore effective as a preventative of mental deficiency and neurologic abnormalities. The diet is less effective in reversing changes which have already developed.

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