Severity assessment scores were first developed to predict the 30 day mortality in community acquired pneumonia; however, several guidelines have extended their use to guide empirical antibiotic prescription decisions. This approach has theoretical advantages because a decrease in broad-spectrum antibiotic treatment in low-risk patients might reduce antibiotic-related side-effects, and to give broad-spectrum therapy to patients at higher risk of death is intuitive. However, evidence in support of this approach is not clear. In particular, the British Thoracic Society guidelines suggest withholding a macrolide from patients with low CURB 65 scores, despite evidence that these patients have a higher frequency of atypical pathogens than do those with a higher severity of pneumonia. Severity scores do not perform well in some groups and might overestimate disease severity in elderly people, leading to inappropriate broad-spectrum treatment to those at high risk of complications such as Clostridium difficile infection. In this Review, we discuss the evidence for antibiotic prescribing guided by severity score and suggest that more evidence of effect and implementation is needed before this approach can be universally adopted.