Patients with chronic lung disease (CLD) have frequent episodes of spontaneous desaturations. Utilizing computerized pulse oximetry (CPO) we quantified the frequency and severity of spontaneous desaturations in very low birth weight (VLBW) infants with CLD. Thirty-four studies by CPO were performed in intubated infants for 4 hours; 17 patients (birth weight, 550-980 g; postnatal age 28-85 days) had CLD, and 17 (birth weight, 520-980 g; postnatal age, 1-7 days) had acute lung disease. Oxygen saturation (SaO2) was measured with the Nellcor N-200 oximeter, its serial output (updated once a second) captured by a computer. Pulse rate, pulse amplitude, and heart rate were also monitored continuously. We measured respiratory system mechanics in 23 patients. Tidal volume (VT), respiratory system compliance (Crs), and resistance (Rrs) were obtained by the PeDS system. Spontaneous desaturation to SaO2 less than 90% occurred for 4.5% of the time in acute patients vs. 27.1% of the time in chronic patients (P less than 0.0001); to SaO2 less than 85%, 0.7% vs. 7.6% of the time in acute vs. chronic patients (P less than 0.002); and to SaO2 less than 80%, 0.4% vs. 2.6% of the time in acute vs. CLD patients (P less than 0.05). Rrs was significantly higher in the ventilated patients with CLD (174 cmH2O/L/s) than in the ventilated patients with acute lung disease (94 cmH2O/L/s, P less than 0.0001). The mean Crs values of the two groups were comparable. Our preliminary data indicate that VLBW infants with CLD receiving assisted ventilation have a greater number of spontaneous desaturation episodes, as compared to patients with acute lung disease.