The physiological abnormalities and clinical correlates of 32 infants consecutively hospitalized with lower respiratory tract disease from respiratory syncytial virus (RSV) were studied in an attempt to characterized the infant most at risk for the acute and long-term complications of RSV infection. Arterial oxygen saturation (Sao2) determinations were obtained daily by means of an ear oximeter. On admission all infants were hypoxemic with a mean Sao2 of 87% (range, 74% to 95%). The mean of the lowest Soa2 recorded during their hospitalization was 85.5% (range, 53% to 96%). The hypoxemia improved little during hospitalization but showed improvement three to seven weeks later. The severity of the hypoxemia correlated significantly with the duration of viral shedding, occurrence of apnea, respiratory rate, age, and percentage of immature neutrophils. Clinical severity did not correlate with the degree of hypoxemia.