OBJECTIVE To determine the frequency of hypoglycemia in patients with type I diabetes and impaired awareness of hypoglycemia by prospective assessment.

RESEARCH DESIGN AND METHODS A prospective study was undertaken for 12 months in 60 patients with type I diabetes: 29 had impaired awareness of hypoglycemia and 31 retained normal awareness of hypoglycemia. The two groups of patients were matched for age, age at onset of diabetes, duration of diabetes, and glycemic control. Episodes of severe hypoglycemia were recorded within 24 h of the event and verified where possible by witnesses.

RESULTS During the 12 months, 19 (66%) of the patients with impaired awareness had one or more episodes of severe hypoglycemia with an overall incidence of 2.8 episodes · patient^–1 ·year^–1. By comparison, 8 (26%) of the patients with normal awareness experienced severe hypoglycemia (P < 0.01) with an annual incidence of 0.5 episode · patient^–1 · year^–1 (P < 0.001). Severe hypoglycemia occurred at different times of the day in the two groups: patients with impaired awareness experienced a greater proportion of episodes during the evening (P = 0.03), and patients with normal awareness experienced a greater proportion in the early morning (P = 0.05). An assessment of fear of hypoglycemia revealed that patients with impaired awareness of hypoglycemia worried more about hypoglycemia than did patients with normal awareness (P = 0.008), but did not modify their behavior accordingly.

CONCLUSIONS This prospective evaluation demonstrated that impaired awareness of hypoglycemia predisposes to a sixfold increase in the frequency of severe hypoglycemia, much of which occurred at home during waking hours.