This article systematically reviews the literature on the outcome of acute hospitalization for adult general psychiatric patients admitted involuntarily as compared to patients admitted voluntarily. Inclusion and exclusion criteria qualified 41 out of 3,227 references found in Medline and PSYNDEXplus literature searches for this review. The authors independently rated these articles on six pre-defined indicators of research quality, carried out statistical comparisons ex-post facto where not reported, and computed for each adequate result the effect size index d for the comparison of means, and the Phi- or contingency coefficient for cross-tabulated data. Methodological quality of the studies, coming mostly from North American and European countries, showed significant variation and was higher concerning service-related than clinical or subjective outcomes. Main deficits appeared in sample size estimation, lack of clear follow-up time-points, and the absence of standardized instruments used to assess clinical outcomes. Length of stay, readmission risk, and risk of involuntary readmission were at least equal or greater for involuntary patients. Involuntary patients showed no increased mortality, but did have higher suicide rates than voluntary patients. Further, involuntary patients demonstrated lower levels of social functioning, and equal levels of general psychopathology and treatment compliance; they were more dissatisfied with treatment and more frequently felt that hospitalization was not justified. Future methodologically-sound studies exploring this topic should focus on patient populations not represented here. Further research should also clarify if the legal admission status is sufficiently valid for differentiating the outcome of acute hospitalization.