A family study has been presented in which two PKU females on non-restricted diets have had 16 abortions, 6 offspring who died in infancy, and 4 presently surviving children. All live births to these females have had varying combinations of premature birth weight, microcephaly, cardiac defects, mental and physical retardation, dislocated hips, and strabismus. Each of the offspring tested was non-phenylketonuric. Thus, adverse effects of elevated maternal blood phenylalanine or associated metabolic abnormalities on the developing fetus are suggested. An additional finding of interest was the presence of normal phenylalanine tolerance tests in the three children of the PKU mothers available for testing. The problem thus presented by reproduction in a PKU mother may become more prevalent with the advent of the many state programs now operating to detect and treat PKU in the neonatal period. From the cases reported previously and those of the present study, it would seem desirable to keep the blood phenylalanine levels of PKU females in the normal range during the child bearing years. Lowering of high levels after the usual delay in diagnosing pregnancy may prove to be inadequate. However, because phenylalanine levels have not yet been controlled in a PKU mother during gestation, there is no proof that such control will preclude the birth of defective offspring. A practical disadvantage associated with the only presently available method of lowering blood phenylalanine, the low phenylalanine diet, is the inability of adults to tolerate the taste.