The objective of this study was to determine whether a favorable effect of short term continuous sc insulin infusion (CSII) therapy on the peripheral and autonomic nervous system could be maintained by subsequent intensified conventional treatment (ICT). Nine type I diabetic patients, aged 18-32 yr, who had been diabetic for 4-23 yr and had reduced nerve conduction velocities received CSII for 4 weeks and subsequently ICT for up to 26 weeks. Motor and sensory nerve conduction velocities (MNCV and SNCV) and heart rate variations during deep breathing (E/I ratio), during lying and standing (30/15 ratio), and during the Valsalva maneuver (Valsalva ratio) were measured before CSII and at intervals of 1, 2, 4, 6, 10, and 26 weeks. During CSII, MNCV and SNCV increased significantly (P less than 0.01), the E/I ratio improved in seven patients (P less than 0.05), the Valsalva ratio increased in eight patients (P less than 0.01), and the 30/15 ratio increased in five patients. The E/I ratio increased significantly earlier than the Valsalva ratio (P less than 0.025). During ICT, nerve conduction velocity slightly, though not significantly, decreased, and the results of the cardiovascular reflex tests also gradually declined. The hemoglobin A1 concentration before initiation of CSII and the diminution of the hemoglobin A1 concentration during CSII therapy were inversely correlated to the increase in MNCV (P less than 0.01 and P less than 0.05, respectively). In conclusion, CSII improved peripheral and autonomic nervous system function, but the improvement diminished somewhat during ICT.