To determine the efficacy of CGS 5649B, the authors conducted a series of randomized, double-masked, single-subject studies using pairs of treatment periods, each pair including four weeks of active drug administration and four weeks of placebo administration. Thirty-two moderately demented patients completed three treatment pairs, receiving doses of 90, 200, or 600 mg of CGS 5649B during the active periods. Two patients met criteria for individual response in the behavioral measures. In the group analysis none of the differences achieved statistical significance. The authors conclude that CGS 5649B had a favorable impact on behavior for two of 32 patients.