To examine the long-term effects of recurrent severe hypoglycaemia and other biomedical complications on mental efficiency, a battery of cognitive tests was administered to 142 Type 1 (insulin-dependent) diabetic adult patients (age 33.5±5.6 years; mean ±SD) and 100 demographically similar non-diabetic control subjects. All diabetic subjects had been diagnosed before the age of 17 years. Diabetic subjects with one or more complications (distal symmetrical polyneuropathy; advanced background or proliferative retinopathy; overt nephropathy; one or more episodes of severe hypoglycaemia) performed significantly (p<0.001) more poorly than non-diabetic control subjects on tests requiring sustained attention, rapid analysis of visuospatial detail, and hand eye co-ordination. Regression analyses indicated that the best biomedical predictor of cognitive test performance was a diagnosis of polyneuropathy. Although severe recurrent hypoglycaemia was not associated with performance on any test, the neuropathy × recurrent hypoglycaemia interaction term was significant. These results suggest that in adults with Type 1 diabetes of long duration, recurrent hypoglycaemia does not appear to influence cognitive performance directly, but may interact with neuropathy to exaggerate or otherwise magnify the extent of neurobehavioural dysfunction.