To assess the occurrence and nature of sleep-disordered breathing (SDB) in 26 adult, nonobese diabetics (18 with autonomic neuropathy (DAN+) (age 45 (41-50) yrs; body mass index (BMI) 24.1 (22-26) kg x m(-2)) and eight without autonomic neuropathy (DAN-) (age 45 (35-55) yrs; BMI 24.8 (23-26) kg x m(-2))) overnight full sleep studies and measurements of central and peripheral carbon dioxide (CO2) chemosensitivity were performed. DAN+ were divided in two subgroups, according to the presence (DAN+PH+; n=10) or absence (DAN+PH-; n=8) of postural hypotension. Ten normal subjects were studied as controls (age 42 (36-48) yrs; BMI 24.4 (23-25) kg x m(-2)). In contrast to DAN- and controls, who did not show SDB, five DAN+ (four DAN+PH- and one DAN+PH+) had an apnoea/hypopnoea index > or = 10 and four DAN+ (two DAN+PH- and two DAN+PH+) had an apnoea index > or = 5. All the events were obstructive, occurring mainly during rapid eye movement (REM) sleep. Ten DAN+ exhibited a mean lowest oxygen saturation < 90% during REM sleep. No periodic breathing or central sleep apnoeas were found in DAN+PH+, although they had an enhanced central chemoresponsiveness to CO2. Both DAN+ subgroups showed a marked reduction in peripheral CO2 chemosensitivity. In conclusion, adult nonobese diabetics with autonomic neuropathy, independent of the severity of their dysautonomy, have obstructive sleep apnoea/hypopnoea with a frequency > 30%. A decrease in peripheral carbon dioxide chemosensitivity prevents adult nonobese diabetics with autonomic neuropathy and postural hypotension from experiencing posthyperventilatory central sleep apnoea, despite an increased hypercapnic central drive.