['To evaluate whether autonomic neuropathy predicts deterioration in glomerular filtration rate in IDDM patients.', 'A prospective study in which 35 IDDM patients have been followed for 10-11 yr. Autonomic nerve function tests included heart-rate reactions to deep breathing (expiration-to-inspiration ratio) and to tilt (acceleration and brake indexes). GFR was evaluated by the 51Cr-EDTA plasma clearance method.', 'At entry to the study, no significant differences were noted in age (39 +/- 2 [mean +/- SE] vs. 42 +/- 4 yr), duration of diabetes (20 +/- 3 vs. 23 +/- 4 yr), supine blood pressures (120/79 +/- 3/2 mmHg vs. 121/78 +/- 6/3 mmHg), and GFR (113 +/- 6 vs. 107 +/- 3 ml.min-1.1.73 m-2) between 20 patients with and 15 without autonomic neuropathy (age-corrected criteria). After 10-11 yr, GFR had decreased significantly (22 +/- 4 ml.min-1.1.73 m-2, P < 0.001) in patients with autonomic neuropathy but not (8 +/- 5 ml.min-1.1.73 m-2, NS) in patients without. In keeping with this, GFR decreased more than expected (difference in GFR/expected decrease in GFR) in patients with autonomic neuropathy, compared with those without (4.46 +/- 0.98 vs. 0.48 +/- 0.73, P < 0.005).'] Does autonomic neuropathy predict deterioration in glomerular filtration rate in patients with IDDM?