The classification accuracy of the Portland digit recognition test (PDRT) in detecting cognitive malingering was studied in patients claiming cognitive deficits due to exposure to environmental or industrial toxins. Twenty-nine patients alleging toxic exposure and who met Slick et al. [Slick, D. J., Sherman, E. M. S., & Iverson, G. L. (1999). Diagnostic criteria for malingering neurocognitive dysfunction: Proposed standards for clinical practice and research. The Clinical Neuropsychologist, 13, 545–561] criteria for malingered neurocognitive dysfunction were compared to 14 toxic exposure patients negative for evidence of malingering. The published cutoffs were associated with a false positive error rate of 0% and sensitivity of more than 50%. When criterion for a PDRT failure was a positive PDRT finding on more than one section, the FP rate remained 0% while sensitivity improved to about 70%. The results indicate that a failed PDRT is an indication of malingering and not the neurological effect of a toxic substance or some other clinical phenomenon. The PDRT can be used with confidence as an indicator of negative response bias in cases of alleged exposure to neurotoxic substances.