Two groups of patients who developed orthostatic hypotension (OH) after spinal cord injury (SCI) were studied. In the first group all patients (4 females and 6 males) were asymptomatic, whereas in the second group (1 female and 9 males) all had clinical manifestations of hypotension. All but 3 patients were tetraplegic, and these patients were paraplegic above the T6 level. For this study blood pressure (BP), heart rate and cerebral blood flow (CBF) velocity were measured simultaneously on a tilt table at 0, 30, 60, and 80 degrees. Cerebral blood flow in the middle cerebral artery was measured bilaterally utilising the transcranial Doppler technique. In asymptomatic patients the mean baseline (0 degrees) BP (110 +/- 16/70 +/- 77 mm Hg systolic/diastolic) was not significantly different from the BP (106 +/- 16/68 +/- 11 mm Hg) of symptomatic patients. The mean maximal change in BP during tilting in the asymptomatic group (-23 +/- 10/10 +/- 7 mm Hg) was also not significantly different when compared to the symptomatic group (-29 +/- 13/11 +/- 6 mm Hg). CBF in the symptomatic group during the hypotensive reaction at 80 degrees was 32.5 +/- 5 cm/sec, while at the same body position in the asymptomatic group it was 40.9 +/- 8 cm/sec (significant at the p less than 0.02). In addition, CBF decreased in the symptomatic group at 80 degrees to 55.5 +/- 9.6% of baseline, while in the asymptomatic group the fall was 69.3 +/- 7.2% (p less than 0.001). Our data suggest that autoregulation of CBF rather than systemic BP plays a dominant role in the adaptation to OH in patients with SCI.