The analysis of the treatment of diaphyseal fractures of tre long bones with functional intraosseous osteosynthesis (FIO) of 593 patients (675 bones) during three years is presented. The techniques of osteosynthesis have been worked out by the author and are performed without using an electro-optical transducer and drilling out of the medullary canal. At the postoperative period no immobilization with a plaster cast is necessary. The function of the operated extremity is restored before the fraction is consolidated. The disability terms are shorter than usual. There have been no non-unions, false joints or contractures. There was one case of osteomyelitis (0.1%). The definition of FIO is given, the conditions for its performance and the requirements for the intraosseous fixator are presented as well as the facts of making use of FIO in the treatment of non-unions and false joints of the long bones are listed.