This study further develops the Diagnostic Survey for Leadership Improvement (DSLI) and determines its reliability and validity. Split-half estimates of the DSLI's reliability were computed. These estimates came in terms of the Pearson product-moment correlation coefficients that were then corrected by the Spearman-Brown Prophesy Formula. The reliability coeffecients exceeded .96 and were significant at the .0001 level in every instance. Content validity was demonstrated in relationship to Rensis Likert's "Profile of Organizational Characteristics." Data results were subjected to a Principal Components Analysis and the obtained factor matrices were clarified through Varimax orthogonal rotation. These results demonstrated construct validity, that is, the items in the DSLI do accurately and adequately support Likert's theoretic structure. A practical utility experiment (construct validity) demonstrated that scores for targeted DSLI items change significantly (0.05 level). The DSLI has added to the available theory development and its accompanying technology. The DSLI is a reliable and valid diagnostic survey that can be used for both research and field leadership improvement purposes. Extensive appendixes present the survey instrument and related data. (Author/IRT)