Proinsulin content was measured in the serum of 82 children (aged from 3 to 14 years) with type 1 diabetes mellitus of various duration. Three groups of patients characterized by low (54%), normal (42%) and high (4%) levels of this prohormone were recognized. No dependence the proinsulin level on the disease term was found. The serum proinsulin level may be used as a parameter specifying the pathogenesis of type 1 diabetes mellitus.