This study estimated classes of children's acute-stay psychiatric acuity trajectories in terms of shape (i.e., linear, quadratic, cubic) and rate of change (slope). A total of 788 children served on three child units (ages 4-12) were studied. The Children's Acuity of Psychiatric Illness (CAPI) was completed each weekday by trained frontline staff on the milieu. Latent class growth analysis was applied to the data, and seven acuity trajectory classes provided the most parsimonious fit. Four classes evidenced a significant quadratic term, one class a significant linear term, and two classes did not evidence a significant change in acuity. The classes varied in survival time to rehospitalization, in pre-treatment community service use and rates of seclusion, restraint, and emergency medications during the episode. Overall, the results suggest that acute-stay patients may have distinct and identifiable psychiatric acuity change patterns during their episodes and that some may experience non-linear (i.e., quadratic) acuity trajectories.