The authors studied the analytic and clinical characteristics of transcutaneous carbon dioxide monitoring in acutely ill NICU infants. Most infants were premature. The authors used both developmental and commercially available equipment. Monitoring was performed at temperature settings of 44 degrees C. With commercially available equipment, it was found that the least-squares linear regression relationship (r = 0.86, Sy.x = 12.9 torr, n = 100 patients) between transcutaneous PCO2 (y) and arterial PCO2 (x) was: y = -2.8 + (1.86)(x) (in torr). Calibration drift was unacceptably large in about 10% of the 500 monitoring sessions performed. Therefore, the authors conclude that transcutaneous PCO2 monitoring is most appropriate for following short-term trends in PCO2 over a period of one to three hours. Continuous transcutaneous PCO2 monitoring was most useful clinically after extubation when the objectives were careful observation for changes in PCO2 and prevention of reintubation.