Cardiovascular autonomic function was assessed in 9 subjects with Raynaud's phenomenon. The underlying diseases were systemic lupus erythematosus (n = 5), systemic sclerosis (n = 3) and rheumatoid arthritis (n = 1). Five standard non-invasive tests, 3 of heart rate and 2 of blood pressure, were employed. Compared with age and sex matched controls (n = 25), the number of values abnormal was 24 of 45 (53%) overall and between one and 4 (median, 2) individually. Significant differences were present for 3 tests, two of heart rate and one of blood pressure. The subjects were given triiodothyronine, 60 to 80 micrograms per day, for vasospastic attacks. Autonomic function was reassessed between weeks 4 and 9 (9 subjects) and between weeks 12 and 18 (8 subjects) after introduction of triiodothyronine. Test results showed a considerable improvement. At the second reassessment, the number of values abnormal was now 5 of 40 (12.5%) overall and nil (n = 4) or one (n = 4) individually. Significant differences remained for one heart rate test only. Adverse side effects to triiodothyronine occurred in a single subject and were readily controlled. Evidence of somatic neuropathy was present electrophysiologically in all 9 subjects and clinically in 8. Triiodothyronine may have corrected autonomic dysfunction by increasing blood flow to ischaemic peripheral nerves or by acting on the autonomic system more directly. Further study of triiodothyronine in autonomic insufficiency appears merited.