Background. We have found no papers evaluating nonadherence to guidelines for the treatment of atrial fibrillation (AF), taking into account the two risk scales [C, congestive heart failure; H, hypertension; A, age; D, diabetes mellitus; S, stroke (CHADS(2)) and C, congestive heart failure; H, hypertension; A, age; D, diabetes mellitus; S, stroke; V, vascular disease; A, age; Sc, sex category (CHA(2)DS(2)-VASc)] and the two types of treatment that are recommended (antiplatelet/anticoagulant therapy). Objective. To determine the extent of lack of adherence when prescribing anticoagulant and antiplatelet therapy to patients with AF and associated factors. Methods. Cross-sectional, observational study of 144 patients with AF who visited the emergency department of Elda Hospital in 2013-14 (Spain). Main variable: the patient was prescribed a therapy different from that indicated by the guidelines (nonadherence) or not prescribed any therapy. Secondary variables: CHADS(2), CHA(2)DS(2)-VASc, HAS-BLED, type of AF and symptoms related to AF. Multivariate models were constructed to identify the associated factors by calculating the adjusted odds ratios (OR). Results. Nonadherence occurred in 90 patients [62.5%, 95% confidence interval (CI): 54.6-70.4%]. Associated factors were higher CHADS(2) (OR = 1.30, 95% CI: 0.96-1.75, P = 0.091) and CHA(2)DS(2)-VASc (OR = 1.23, 95% CI: 1.02-1.47, P = 0.027), and lower HAS-BLED (OR = 0.67, 95% CI: 0.49-0.91, P = 0.011). Conclusions. Nonadherence to guidelines was found in three out of every five patients. A greater cerebrovascular risk and a lower haemorrhagic risk were associated with this behaviour. Qualitative studies are needed to determine the causes.