Hyperglycinemia is a non rarely observed biochemical finding which can be caused by a primary defect of the glycine cleavage system (nonketotic hyperglycinemia) or by an enzymatic block due to toxic metabolites (ketotic hyperglycinemia in organic acidurias) or to specific drugs (such as sodium valproate). The Authors report their clinical and laboratory experience in the diagnosis of different forms of hyperglycinemia. Three patients with nonketotic hyperglycinemia (1 patient with transient neonatal form, 1 with the classic neonatal form, and 1 with late onset form), 6 patients with ketotic hyperglycinemia (1 propionic acidemia, 5 methylmalonic acidurias), and 8 patients with hyperglycinemia during sodium valproate treatment are described. In this study the diagnostic iter and the importance of a precise diagnosis of the hyperglycinemia for the appropriate treatment are discussed.