The diagnosis of nonketotic hyperglycinemia is considered to depend upon the presence of increased cerebrospinal fluid glycine and an increased cerebrospinal fluid to plasma glycine ratio. We studied two siblings who have the neurologic and peripheral biochemical features of the atypical variant of nonketotic hyperglycinemia but have normal cerebrospinal fluid glycine and cerebrospinal fluid to plasma glycine ratios. The proband had reduced liver glycine cleavage system activity of 17% and 21% of mean normal values, confirmed in two independent laboratories. Her lymphoblast glycine cleavage system activity was normal. Nonketotic hyperglycinemia can be present in the absence of increased cerebrospinal fluid glycine. Measurement of liver glycine cleavage system activity is indicated when nonketotic hyperglycinemia is suggested by clinical features and peripheral glycine levels but cerebrospinal fluid glycine is normal.