Acute diabetic emergencies are potentially avoidable or amenable to timely and effective outpatient therapy. The objective of this study was to evaluate the relationship between socioeconomic status (SES) and acute complications of diabetes mellitus in Ontario. This study used a population-based cohort of persons with diabetes mellitus (N = 605 825) derived from hospital and physician service claims between April 1, 1992, and March 31, 1999. Socioeconomic status was estimated using neighborhood-level data from the 1996 Canadian Census. Outcome events were defined as 1 or more hospitalizations or emergency department visits for hyperglycemia or hypoglycemia. There was a clear inverse gradient between income level and event rates. Individuals in the lowest income quintile were 44% more likely to have an event than those in the highest quintile (16.4% vs 11.4%; P<.001) and had a greater propensity toward recurrent emergency department admissions (1.9 vs 1.6 episodes per patient; P<.001). The gradient was most marked in 45- to 64-year-olds (odds ratio [OR], 1.76) and less apparent in children (OR, 1.06). The relationship between SES and events persisted after adjusting for age, sex, urban vs rural residence, comorbidity, frequency of physician visits, continuity of care, physician specialty, and geographic region (adjusted OR, 1.09] per quintile level). In contrast, admission rates for non-ambulatory care-sensitive conditions (appendicitis and hip fracture) were unaffected by SES. Even when some economic barriers to accessing care are removed, patients from low-SES neighborhoods still experience an excess number of hospitalizations for conditions that should be prevented by optimal care in the ambulatory setting.