This study investigated predictors of length of stay (LOS) for two inpatient psychiatric units (a crisis stabilization unit and a longer-stay milieu unit) in a large, urban, university-affiliated, county hospital. It was hypothesized that three variables would be independently associated with shorter LOS: (1) higher Global Assessment of Functioning (GAF) scale scores, (2) not requiring the use of seclusion or restraints, and (3) the presence of a comorbid substance use disorder. Data were collected on consecutive discharges (n=234) from the two inpatient units. Bivariate tests were conducted, and multiple linear regression models assessed the independent effects of potential determinants of LOS. The presence of a comorbid personality disorder or substance use disorder was predictive of shorter LOS on the crisis stabilization unit (n=88). Several variables were found to be predictive of shorter LOS on the longer-stay milieu unit (n=146), including: involuntary legal status on discharge, not requiring seclusion or restraints, higher admission GAF scale score, female gender, and the presence of a comorbid substance use disorder. Findings indicate that a substantial portion of the variance in LOS in this setting can be predicted from basic sociodemographic and clinical factors available in hospital medical charts.