To examine the association between reasoning through medical treatment decisions and cognition in a sample of patients with brain metastasis. The association between reasoning and cognition was examined using data from 41 patients with diagnosed brain metastasis. All diagnoses were made by a board-certified radiation oncologist and were verified histologically. In total, 41 demographically matched, cognitively healthy controls were also included to aid in classifying patients with brain metastasis according to reasoning status (i.e., intact or impaired). Results indicate that measures of episodic memory and processing speed were associated with reasoning. Using these two predictors, actuarial equations were constructed that can be used to help screen for impaired reasoning ability in patients’ with brain metastasis. The equations presented in this study have clinical significance as they can be used to help identify patients at risk for possessing a diminished ability to reason through medical treatment decisions and, thus, are in need of a more comprehensive evaluation of their medical decision-making capacity. (JINS, 2015, 21, 412–418)