Background The question of whether gender-related disparities still exist in the treatment and outcomes of patients presenting with acute coronary syndromes (ACS) remains controversial. Using data from 4 registries spanning a decade, we sought to determine whether sex-related differences have persisted over time and to examine the treating physician's rationale for adopting a conservative management strategy in women compared with men. Methods From 1999 to 2008, 14,196 Canadian patients with non–ST-segment elevation ACS were recruited into the Acute Coronary Syndrome I (ACSI), ACSII, Global Registry of Acute Coronary Events (GRACE/GRACE2 ), and Canadian Registry of Acute Coronary Events (CANRACE) prospective multicenter registries. Results Women in the study population were found to be significantly older than men and were more likely to have a history of heart failure, diabetes, or hypertension. Fewer women were treated with thienopyridines, heparin, and glycoprotein IIb/IIIa inhibitors compared with men in GRACE and CANRACE. Female gender was independently associated with a lower in-hospital use of coronary angiography (adjusted odds ratio 0.76, 95% CI 0.69-0.84, P < .001) and higher in-hospital mortality (adjusted odds ratio 1.26, 95% CI 1.02-1.56, P = .036), irrespective of age ( P for interaction =.76). Underestimation of patient risk was the most common reason for not pursuing an invasive strategy in both men and women. Conclusions Despite temporal increases in the use of invasive cardiac procedures, women with ACS are still more likely to be treated conservatively, which may be due to underestimation of patient risk. Furthermore, they have worse in-hospital outcomes. Greater awareness of this paradox may assist in bridging the gap between current guidelines and management practices.