Cognitive processes in a group of neurologically asymptomatic patients with relatively severe but uncomplicated insulin-dependent diabetes mellitus (IDDM) were studied. In comparison with a homogeneous group of normoglycemic controls, the diabetic group performed significantly worse in global memory, abstract reasoning, and eye-hand coordination tests. The two groups scored similarly in intelligence, concentration and attention, spatial, visual, and psychomotor tests. The neuropsychological deficits did not correlate with the duration or the severity of the disease. Whether these mild neuropsychological deficits are transient or stable or whether they are caused by central nervous system vascular or metabolic dysfunctions or by the emotional influence of the chronic illness on the intellectual and educational development of patients remains unclear. Our findings need to be cautiously interpreted and perhaps could not be extended to diabetic patients with better metabolic control.