Background: We sought to develop and validate an excitement subscale from the Positive and Negative Syndrome Scale (PANSS) to allow the investigation of mania-like excitement symptoms in clinical trials of patients with schizophrenia using the PANSS and to provide clinicians with a short assessment tool for these states. Methods: Baseline PANSS data from six double-blind, randomized registration trials of olanzapine, three in schizophrenia and three in acute bipolar mania, were used in these post-hoc analyses. Schizophrenia study data were pooled and randomly split in half. Exploratory principal component factor analysis was performed on half of the data. Factors were extracted based on minimum eigenvalue criteria (eigenvalue≥1); loadings were determined using an equamax rotation. Confirmatory principal component factor analysis was performed on the other half of the data, retaining the original number of factors. Principal component factor analysis was also done for the pooled bipolar studies. Change in the new mania-like factor scores was then correlated with Young Mania Rating Scale (Y-MRS) scores in each bipolar study. Results: Exploratory principal components analysis on the pooled schizophrenia data extracted five factors: negative, positive, excitement, cognitive, and depressive factors. The mania-like excitement factor was represented by four items (uncooperativeness, poor impulse control, excitement, and hostility), with only moderate loadings by tension and suspiciousness/persecution. Results were similar in the confirmatory analysis and the pooled bipolar studies. Change from baseline to endpoint for the mania-like factor correlated reasonably well (0.64–0.78) with change in Y-MRS scores in the bipolar studies. At baseline, bipolar patients scored higher than patients with schizophrenia on three of four PANSS mania-like factor items: poor impulse control, excitement, and hostility; the converse was true for most other PANSS items. Conclusion: Factor analyses of the PANSS consistently uncovered an excitement factor including uncooperativeness, poor impulse control, excitement, and hostility items. This factor may be useful in examining manic symptoms in studies where the addition of a scale specific to mania would be burdensome and where symptoms of excitement are part of the clinical presentation.