A syndrome has been observed in a kindred with deficient activity of hypoxanthine-guanine phosphoribosyltransferase in which affected hemizygotes have had mild mental retardation, a spastic gait, pyramidal tract signs, shortness of stature, proximally placed thumbs and clinodactyly of the fifth fingers. Activity of the enzyme was virtually zero in lysates of erythrocytes or hair roots, but in intact fibroblasts, the level of activity was 7.5% of normal, placing this variant in a group distinct from any previously studied. Kinetic studies revealed a Michaelis constant for hypoxanthine that was also different from other enzymes studied. These observations indicate the presence in this kindred of a distinct variant of hypoxanthine-guanine phosphoribosyltransferase.