Measurement of central motor conduction time (CMCT) after percutaneous magnetic stimulation of the brain is an electrophysiological method that may discover subclinical impairment of central nervous system (CNS). In order to detect an impairment of CNS, we measured CMCT right (R) and left (L) after percutaneous stimulation of the brain in 34 patients affected by insulin-dependent diabetes mellitus (IDDM) (16 males and 18 females), aged 16.4 +/- 4.1 years (7.3-23.2 years), with duration of disease 7.6 +/- 4.9 years (7/12-16 years), and HbA1c annual mean 7.41 +/- 1.1% (n.v. 5.14 +/- 0.84%). Twenty-three sex- and age-matched healthy subjects served as controls. In our IDDM patients we observed a delay of CMCT R (P < 0.0005) and L (P < 0.0005) as compared to controls. No correlation was found between CMCT (R and L) and chronologic age, duration of disease, peroneal motor nerve conduction velocity. No association was observed between CMCT (R and L) and HLA antigens. On the basis of IDDM duration, patients were divided into 2 groups (G): G I (9 pts) with IDDM < 2 years and G II (25 pts) with IDDM > 5 years, 12 of them with precocious signs of one or more microangiopathic complications. No difference in CMCT (R and L) was observed between the 2 groups and between G I and controls; G II patients had a longer delay of CMCT R (P < 0.0001) and L (P < 0.0001) than controls. In G II patients, a positive correlation between CMCT R and HbA1c of the 5 years before the test (P < 0.025) was also observed.(ABSTRACT TRUNCATED AT 250 WORDS)