A series of seven psychometric tests, to evaluate mental concentration and the ability to retain selective attention, lexical fluency, wordlist memorizing and psychomotor speed, was performed on 25 non-diabetic control subjects and 55 insulin-dependent diabetes (IDD) patients of similar social background and professional status. When tested, none of the diabetics was hypoglycaemic and these patients were divided into two groups: Group I: 30 IDD patients unaware of hypoglycaemia, and experiencing frequent and severe episodes of hypoglycaemia. Group II: 25 IDD patients aware of hypoglycaemia. Groups I and II had experienced the disease for the same period of time (17 +/- 13 vs. 14 +/- 11 years, respectively) and they had similar HbA1c levels (7.14 +/- 1.25% vs. 8.6 +/- 1.88%, respectively) and degenerative complications. Compared with the scores of the controls, the Group I scores were lower in four tests: trail-making part A (psychomotor speed; P less than 0.001) and part B (retaining selective attention; P less than 0.01), lexical fluency (P less than 0.01) and Rey auditory-verbal learning test (wordlist learning; P less than 0.05). Group II scores were lower in two tests: trail-making part A (P less than 0.01) and part B (P less than 0.05). In word memorizing, the performance of Group I was inferior to that of Group II (P less than 0.05). In general, these psychometric tests showed that IDD scores were lower than those of the controls, with an average of 67% for Group II and 80% for Group I. Chronic hyperglycaemia and severe hypoglycaemia may have a deleterious effect on cognitive performance. In particular, several severe episodes of hypoglycaemia could be responsible for permanent memory impairment.