Purpose: To estimate the proportion of patients presenting with isolated third, fourth, or sixth cranial nerve palsy of presumed microvascular origin versus other causes. Design: Prospective, multicenter, observational case series. Participants: A total of 109 patients aged 50 years or older with acute isolated ocular motor nerve palsy. Testing: Magnetic resonance imaging (MRI) of the brain. Main Outcome Measures: Causes of acute isolated ocular motor nerve palsy (presumed microvascular or other) as determined with early MRI and clinical assessment. Results: Among 109 patients enrolled in the study, 22 had cranial nerve III palsy, 25 had cranial nerve IV palsy, and 62 had cranial nerve VI palsy. A cause other than presumed microvascular ischemia was identified in 18 patients (16.5%; 95% confidence interval, 10.7-24.6). The presence of 1 or more vasculopathic risk factors (diabetes, hypertension, hypercholesterolemia, coronary artery disease, myocardial infarction, stroke, and smoking) was significantly associated with a presumed microvascular cause (P = 0.003, Fisher exact test). Vasculopathic risk factors were also present in 61% of patients (11/18) with other causes. In the group of patients who had vasculopathic risk factors only, with no other significant medical condition, 10% of patients (8/80) were found to have other causes, including midbrain infarction, neoplasms, inflammation, pituitary apoplexy, and giant cell arteritis (GCA). By excluding patients with third cranial nerve palsies and those with GCA, the incidence of other causes for isolated fourth and sixth cranial nerve palsies was 4.7% (3/64). Conclusions: In our series of patients with acute isolated ocular motor nerve palsies, a substantial proportion of patients had other causes, including neoplasm, GCA, and brain stem infarction. Brain MRI and laboratory workup have a role in the initial evaluation of older patients with isolated acute ocular motor nerve palsies regardless of whether vascular risk factors are present. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article.