Only marginally improved results have been observed in standard autonomic function tests (AFT) in follow-up studies after simultaneous pancreas and kidney transplantation (SPK). We therefore used power spectral analysis (PSA) of heart rate variability (HRV) to assess the effect of SPK on autonomic neuropathy in patients with type I diabetes mellitus (DM I). We evaluated 82 patients with DM I who were insulin and dialysis free following SPK. Both pre- and posttransplant (at [mean +/- SD], 25 +/- 15 months post-SPK) examinations were performed in 29 patients. Posttransplant evolution was examined in another 60 patients with two serial examinations at 20 +/- 20 and 43 +/- 27 months after SPK. Comparisons included 32 age-matched healthy controls and 13 patients with kidney transplant alone (KTA) matched for age and duration of DM I at a comparable time point posttransplant. Short-term time (modified Ewing battery) and frequency domain (PSA of HRV: LF-low, HF-high frequency, and TP-total spectral power) analysis was performed with a telemetric, on-line, computer-aided system. Significantly worse results in all standard AFT and PSA indexes were obtained for SPK patients compared with controls at all time points. No significant improvement was seen in SPK patients in the posttransplant period and no differences were found compared with KTA patients. Thus the results of a power spectral analysis of HRV failed to show improvement following SPK. This examination adds little positive information to that obtained from standard autonomic function tests.