To evaluate the longest hospitalizations in an acute psychiatric ward [Service of Psychiatric Diagnosis and Treatment (SPDT)] and the related demographic, clinical and organizational variables to understand the factors that contribute to long-stay (LOS) phenomenon. The term "long stay" indicates clinical, social and organizational problems responsible for delayed discharges. In psychiatry, clinical severity, social dysfunction and/ or health-care system organization appear relevant factors in prolonging stays. Patients and Methods: We divided all the SPDT hospitalizations from 1 January 2010 to 31 December 2015 into two groups based on the 97.5 th percentile of duration: ≤36 day (n=3254) and >36 day (n=81) stays, in order to compare the two groups for the selected variables. Comparisons were made using Pearson's chi-square for categorical data and t-test for continuous variables, the correlation between the LOS, as a dependent variable, and the selected variables was analyzed in stepwise multiple linear regression and in multiple logistic regression models. Results: The longest hospitalizations were significantly related to the diagnosis of "schizophrenia and other psychosis" (Pearson Chi 2 =17.24; p=0.045), the presence of moderate and severe aggressiveness (Pearson chi 2 =29; p=0.000), compulsory treatment (Pearson Chi 2 =8.05; p=0.005), parenteral or other route administration of psycho-pharmacotherapy (Pearson Chi 2 =12.91; p=0.007), poli-therapy (Pearson Chi 2 =6.40; p=0.041), complex psychiatric activities (Pearson Chi 2 =12.26; p=0.002) and rehabilitative programs (Pearson Chi 2 =37.05; p=0.000) during the hospitalization and at discharge (Pearson Chi 2 =29.89; p=0.000). Many demographic and clinical variables were statistically significantly correlated to the LOS at our multiple linear and logistic regression model. Conclusion: In our sample, clinical illness severity and need for complex therapeutic and rehabilitative treatments were associated with prolonged psychiatric hospitalizations. Understanding this phenomenon can have not only economic but also clinical, ethical and social relevance.