It has not been adequately evaluated whether the outcome of infections differs by body-weight category. We performed a systematic review of relevant studies. Eleven studies (one retrospective and 10 prospective cohort studies) were included in this review, involving a total of 3159 hospitalized patients or nursing home residents. Most studies (6/11) referred to lower respiratory tract infections. Seven studies showed an association of patient outcome (mortality in 6/7 studies) with body-weight category. This was shown in multivariate analysis in 4/5 studies that reported relevant data. Obese or morbidly obese patients with infections had worse outcome compared with the rest of the patients or with normal-weight patients, in 4/7 studies that reported relevant data; findings were not significant in the remaining three studies. Patients in the lowest body mass index (BMI) group had worse outcome compared with all other groups combined, in 3/5 studies that reported relevant data; findings were not significant in the remaining two studies. Low BMI was associated with worse outcome in patients with lower respiratory tract infections in 3/4 relevant studies. Although not consistently reported, an association of both ends of the BMI distribution with worse outcome of infections is plausible and merits further investigation.