Neonatal nonketotic hyperglycinemia is usually fatal or, less commonly, severely developmentally disabling, whereas transient nonketotic hyperglycinemia has usually been followed by normal development. We report a boy who had transient neonatal nonketotic hyperglycinemia but a coexistent disorder of serotonin metabolism manifested by initially low cerebrospinal fluid 5-hydroxyindoleacetic acid (which later normalized), low whole blood serotonin, and decreased platelet serotonin uptake. He survived the neonatal period but was neurodevelopmentally delayed and developed an autistic-like disorder. Later, his positron emission tomographic (PET) scans with alpha[(11)C] methyl-l-tryptophan revealed a pattern characteristic of autistic children. Although we know of no link between glycine and serotonin metabolism, and our patient had low, rather than high, central and peripheral serotonin, this case might represent a novel infantile disorder that affects both the glycine and serotonin neurotransmitter systems.