Transtelephone monitoring (TTM) is capable of detecting pacemaker pulse generator malfunction, battery depletion, and lead failure. The accuracy of TTM was analyzed by a review of Montefiore Medical Center records between October 1981 and March 1994. Each group of transmissions from a single patient, starting with implant and ending with a pacemaker operation, was defined as a closed cycle (CLOSE), if undergoing continuing follow-up at the time of analysis, as a continuing cycle (CONT), and if a cycle had ended with death or loss to follow-up, an open cycle (OPEN). TTM records of 2,632 patients were analyzed, providing 3,291 cycles. There were 731 CONT, 433 CLOSE, and 2,127 OPEN cycles; 331 procedures were indicated by TTM, of which 279 were impending depletion, 30 sudden depletion, and 22 lead malfunctions. Of the 102 procedures not indicated by TTM, 85 were for nonurgent reasons (recall: 41; DDD upgrade: 16; patient/MD request: 28) and 17 for urgent reasons. In patients followed by TTM who had a lead problem, 22 were detected by TTM before clinical manifestations and 16 were not. There were no cases in which TTM follow-up did not detect battery depletion. The total number of TTM contacts, available for 3,094 cycles, was 88,654 (range, 1-163, median 19), of which 0.4% yielded a procedure. During the same period, 75% of all secondary interventions during the first 2 years occurred during the first 2 months after implant because of lead malfunction, with a subsequent SI rate of 0.005 per month for the third through the twenty-fourth months.