BACKGROUND: Guidelines recommend evaluation of cardiac function, valvular and ischemic heart disease, and thyroid, kidney, and liver function on initial diagnosis of atrial fibrillation (AF). HYPOTHESIS: We hypothesized that initial workup of patients with newly identified AF would vary by age, sex, and burden of comorbid illness. METHODS: In a retrospective analysis of a large sample of commercially insured patients 18 to 64 years old (n = 40 245) and a nationally representative 5% cohort of Medicare beneficiaries 65 years or older (n = 204 676), we measured claims for guideline-recommended services for initial evaluation of AF among patients with a new diagnosis between 2000 and 2008. RESULTS: From 30 days before through 90 days after AF diagnosis, basic evaluation, including physician visit, electrocardiogram, and echocardiography, was completed in up to 66.6% of patients. Completion rates for all guideline-recommended evaluations were 17.4% in the commercially insured sample and 18.5% in the Medicare cohort in 2007. Evaluation rates increased over time. Blood tests assessing thyroid function were documented for approximately one-third of patients in each cohort. Increasing the observation period to 1 year before through 3 months after the AF diagnosis markedly increased completion rates, but rates of thyroid function testing remained low (50%-60%). There were minor differences in evaluation completeness by sex, race, and geographic region. CONCLUSIONS: Differences in guideline-recommended evaluation rates by demographic characteristics after a new diagnosis of AF were of minor clinical importance. Basic evaluation had satisfactory completion rates; however, rates of laboratory testing were low.