It has been previously shown that boundary layer or flow separation occurring in the carotid bulb and detected by duplex scanning denotes minimal or no carotid atherosclerotic disease as demonstrated by angiography and reliably predicts aetiology other than carotid artery disease in symptomatic patients. To evaluate outcome at long-term follow-up we prospectively studied 94 patients (48 males, 46 females) who demonstrated bilateral flow separation. Mean age was 61.2 years (27 to 86 years). Mean follow-up was 57 months (5 to 113 months). There was one death during follow-up at 69 months. It was stroke related. Using age and sex specific death rates for the general population 14.3 deaths would be expected for the same average period. By life table analysis, survival was 98.7% at five years compared to a general population expected 5 year survival of 85.9%. There were no strokes at 5 years of follow-up. (Age and sex specific stroke-free survival for Rochester, MN 1970-1974 is 98% at 5 years). TIA-free survival was 99% at one year (n = 87) and 96% at five years (n = 46). It is concluded that the presence of boundary layer separation in the carotid bulb not only indicates absent or minimal atherosclerotic disease, but is predictive of a favourable long-term outcome with respect to mortality and neurological events.