The medical records of 163 neonatal foals that had thoracic radiographs taken within 48 hours of admission to a referral hospital were reviewed. The objectives of this study were (1) to identify risk factors for the development of thoracic radiographic changes and (2) to identify prognostic indicators for survival in foals with radiographic evidence of pulmonary disease. Failure of transfer of passive immunity (IgG concentration < or = 400 mg/dL) was the only risk factor for radiographic evidence of respiratory disease identified by multivariate analysis. Hypoxemic patients (PaO2 < or = 60 mm Hg) were 4.9 times more likely to reveal radiographic abnormalities in a subset of foals for which arterial blood gas results were available. Foals with a serum creatinine concentration > 1.7 mg/dL upon presentation, dyspnea, and a history of dystocia were significantly more likely to die based on the multivariate statistical outcome analysis. An anion gap > or = 20 mEq/dL was strongly associated with nonsurvival in a subset of foals with arterial blood gas results. These hematologic and biochemical variables can be readily obtained during the initial evaluation of sick foals. The presence of a high anion gap appeared to have the greatest clinical impact and may be a useful prognostic indicator in foals with radiographic evidence of respiratory disease. In contrast, the majority of physical examination variables, including evaluation of tachypnea, abnormal respiratory sounds, fever, weakness, and milk reflux from the nares, which are usually obtained during the general respiratory evaluation of foals, were unrelated to outcome.