The Category Test (CT) and the Wisconsin Card Sort Test (WCST) have long been the major instruments used for the assessment of "frontal lobe" brain damage. These tests have often been used interchangeably by clinicians. However, research to date has disagreed on the extent to which these tests can be interpreted in a similar manner. The current paper examined the interrelationships between the tests in 112 mixed brain injured clients. The results showed relatively small correlations between the tests in the range of .4 to .6 which were eliminated when age, education, premorbid IQ (Vocabulary) and spatial skills (Block Design) were used as covariates. Regression analyses showed clear differences in which WAIS-R tests predicted each test, while factor analysis revealed little overlap between the tests except for a small relationship between the number of WCST categories completed and CT errors. The CT appeared to reflect spatial and spatial analysis skills while the WCST reflected verbal analysis and sequential skills. The results clearly indicate that the tests are not interchangeable and do not reflect the same underlying cognitive skills nor can they be interpreted in the same manner for neuropsychological purposes.