The testament definition scale (TDS) is a specifically designed six-item scale aimed at measuring the respondent's capacity to define "testament." We assessed the reliability and validity of this new short scale in 31 community-dwelling cognitively impaired elderly patients. Interrater reliability for the six items ranged from .87 to .97. The interrater reliability for the total score was .77. Significant correlations were found between the TDS score and the Mini-Mental State Examination (MMSE) and the Cambridge Cognitive Examination scores (r = .71 and .72 respectively, p = .001). Criterion validity yielded significantly different means for subjects with MMSE scores of 24-30 and 0-23: mean 3.9 and 1.6 respectively (t(20) = 4.7, p = .001). Using a cutoff point of 0-2 vs. 3+, 79% of the subjects were correctly classified as severely cognitively impaired, with only 8.3% false positives, and a positive predictive value of 94%. Thus, TDS was found both reliable and valid. This scale, however, is not synonymous with testamentary capacity. The discussion deals with the methodological limitations of this study, and highlights the practical as well as the theoretical relevance of TDS. Future studies are warranted to elucidate the relationships between TDS and existing legal requirements of testamentary capacity.