Insulin resistance and metabolic dysfunction have been associated with asthma risk and asthma severity.To examine the association between glycated hemoglobin A1c (HbA1c), asthma-related hospitalizations, and lung function measures among adults in the United Kingdom (U.K.).Cross-sectional study of 47,606 adults aged 40 to 69 years old who participated in the UK Biobank and had asthma but no diagnosis of diabetes mellitus. HbA1c level was analyzed as a continuous measure and also categorized as normal (<42 mmol/mol) or as consistent with pre-diabetes/diabetes (≥ 42 mmol/mol). An asthma-related hospitalization was defined as ever having had a hospitalization with an International Classification of Diseases Clinical Modification (ICD) code of a main diagnosis compatible with asthma (ICD-9: 493.x or ICD-10: J45.x and J46.x). Logistic or linear regression was used for the multivariable analysis of asthma hospitalizations and lung function measures (FEV, FVC, and FEV/FVC). All models were adjusted for age, sex, ethnic background, body mass index, average annual household income, current smoking status, pack-years of smoking, fasting time, and C-reactive protein.Both HbA1c level (odds ratio [OR] = 1.03, 95% confidence interval [CI] = 1.01 to 1.04) and a HbA1c in the pre-diabetic or diabetic range (OR = 1.68, 95% CI = 1.18 to 2.41) were associated with ≥1 asthma hospitalization. Moreover, both HbA1c level and a HbA1c in the prediabetic/diabetic range were significantly and inversely associated with FEV and FVC.HbA1c is linked to asthma-related hospitalizations and small decrements in FEV and FVC among British adults with asthma but no diagnosis of diabetes mellitus.