Attention deficits are nearly ubiquitous after traumatic brain injury (TBI). In the subacute phase of moderate to severe TBI, these deficits may be difficult to measure with the precision needed to predict outcomes, assess degree of recovery, and monitor treatment response. This article reports the findings of four studies, three observational and one a randomized, controlled treatment trial of methylphenidate (MP), designed to provide construct validation of the Moss Attention Rating Scale (MARS), an observational measure of attention dysfunction following TBI. One hundred seven participants with moderate to severe TBI were enrolled during treatment on an inpatient rehabilitation unit. MARS scores were provided independently by four rehabilitation disciplines (Physical, Occupational and Speech Therapies and Nursing). Results indicated that the MARS: (1) is more strongly related to concurrent measures of cognitive versus physical disability, supporting its validity as a measure of cognition, (2) is more strongly related to concurrent psychometric measures of attention versus measures thought to rely less on attention, supporting its validity as a measure of attention; and (3) predicts 1-year outcomes of TBI better than psychometric measures of attention. However, the MARS (4) was not differentially affected by MP versus placebo treatment. Results support the construct validity and utility of the MARS, with further research needed to clarify its role in treatment outcome assessment.