Objective: Research on the utility of tests of premorbid IQ yields inconsistent results and there have been no direct comparisons between the Advanced Clinical Solutions (ACS) Test of Premorbid Functioning (TOPF) and Lezak's (1983) “best performance method.” It is hypothesized that the TOPF and the “best performance method” will yield estimated premorbid IQ that is either equal to or greater than the Wechsler Adult Intelligence Scale-IV (WAIS-IV) full-scale IQ (FSIQ). Method: The clinical sample consisted of veterans (N = 59) in an outpatient neuropsychology clinic. The “best performance method” was calculated using the highest WAIS-IV subscale scores. A Wilcoxon Signed Rank Test compared the scores of estimated premorbid IQ using the TOPF and the “best performance method.” Results: 35% of patients had a TOPF score that was below their WAIS-IV FSIQ (p = 0.021), while only 1.69% of patients had a “best performance method” score below the WAIS-IV FSIQ (p < 0.001). The “best performance method” may have overestimated premorbid functioning. TOPF had more variability in estimation, but underestimated at a higher rate. Conclusion: Results do not support the hypothesis and instead suggest that the TOPF may underestimate premorbid IQ, while the “best performance method” may overestimate premorbid IQ. As measures of premorbid IQ are designed to estimate baseline cognitive ability prior to the onset of cognitive decline, it is expected that scores will be equal to or greater than current overall FSIQ. However, further research is needed to determine whether the Lezak's “best performance method” is more clinically useful in a veteran population relative to the TOPF.