There are few available data to define the medically necessary duration of stay for patients hospitalized with pneumonia. Therefore, we investigated the safety and effectiveness of a practice guideline that provided information about switching patients from parenteral to oral antimicrobials and early hospital discharge. The study was a prospective controlled study with an alternate month design. The practice guideline was studied in 146 "low-risk" pneumonia patients hospitalized during a 22-month period. Medical care consistent with the practice guideline occurred in 64% and 76% of patients during control and intervention periods, respectively (p=0.15). There were no differences in patient outcomes in the control and intervention groups when measured 1 mo after hospital discharge, including hospital readmission rates, health-related quality of life, and patient satisfaction. Explicit and implicit review revealed that 98.6% (95% confidence interval [CI]: 95.1%, 99.8%) of low-risk patients would not have benefited from continued hospitalization after the fourth hospital day. The 30-d survival rate of the low-risk pneumonia patients was 99.3% (95% CI: 96.2%, 100%) and patient outcomes appeared to be favorable compared with previously published values. We conclude that duration of hospital stay was frequently consistent with the practice guideline in both study groups, and patient outcomes remained unchanged. The guideline will require additional testing before it can be recommended for use.