OBJECTIVE - Severe hypoglycemia is a major problem for patients with type 1 diabetes and their relatives. The aim of this study was to compare patients' and relatives' assessments of rates of severe hypoglycemia and state of awareness and to explore the influence on involvement and concern of relatives. RESEARCH DESIGN AND METHODS - A cross-sectional paired questionnaire survey on hypoglycemia and the state of awareness was used in our study comprising 284 unselected adult patients with type 1 diabetes and their closest cohabitant. The cohabitant questionnaires also addressed involvement and concern. RESULTS - The agreement between assessments of rates of severe hypoglycemia and state of awareness made by patients and cohabitants was weak (κ 0.404 and 0.442, respectively; P < 0.001). Cohabitants recalled more episodes of severe hypoglycemia than patients (2.7 vs. 1.6 episodes/patient-year; P < 0.001). Degree of involvement was positively related to the rate of severe hypoglycemia (P = 0.002) and negatively related to the state of awareness (P = 0.007) but not to level of HbA1c, duration of diabetes, or presence of late complications, except for peripheral neuropathy (P = 0.01). CONCLUSIONS - Cohabitants of patients with type 1 diabetes recall significantly more episodes of severe hypoglycemia than the patients: The rate of severe hypoglycemia and state of hypoglycemic awareness are the principal determinants of degree of cohabitants' involvement in their partners' disease.