Background. Noroviruses (NoVs) are the most common cause of epidemic gastroenteritis; however, the relative impacts of individual factors underlying severe illness are poorly understood. This report reviews published NoV outbreak reports to quantify hospitalization and mortality rates and assess their relationship with outbreak setting, transmission route, and strain. Methods. Using a string of terms related to "norovirus" and "outbreak," we 2435 nonduplicate articles identified in PubMed, EMBASE, and Web of Knowledge published between January 1993 and June 2011. Inclusion criteria included outbreaks with a minimum of 2 ill persons with a common exposure and at least 1 reversetranscription polymerase chain reaction-confirmed case of NoV disease. Univariate analyses were performed, and multivariable models were fitted to estimate the independent effect of each factor. Results. We analyzed 843 NoV outbreaks reported in 233 published articles from 45 countries. Based upon 71 724 illnesses, 501 hospitalizations, and 45 deaths, overall hospitalization and mortality rates were 0.54% and 0.06%, respectively. In multivariate analysis, genogroup 2 genotype 4 (GII.4) NoV strains were associated with higher hospitalization (incidence rate ratio IRR, 9.4; 95% confidence interval CI, 6.1-14.4; P < .001) and mortality rates (IRR, 3.1; 95% CI, 1.3-7.6; P = .01). Deaths were much more likely to occur in outbreaks occurring in healthcare facilities (IRR, 60; 95% CI, 6-109; P = .01). Conclusions. Our review suggests that hospitalizations and deaths were more likely in outbreaks associated with GII.4 viruses, independent of other factors, and underscores the importance of developing vaccines against GII.4 viruses to prevent severe disease outcomes.