Eighteen ventilator-dependent preterm infants with hyaline membrane disease were studied for 24 hours before and after an attempt at extubation. All were treated with theophylline prior to weaning and achieved average levels of 8.9 +/- 1.7 micrograms/ml (49 +/- 9 mumol/liter) in 13 successfully weaned infants and 8.4 +/- 1.1 micrograms/ml (47 +/- 6 mumol/liter) in 5 infants not extubated, p > 0.05. Infants successfully weaned were significantly (p < 0.01) older, more mature (29 +/- 1 versus 26 +/- 2 weeks' gestational age) and heavier (1107 +/- 236 versus 1016 +/- 256 gm) than infants not successfully extubated. Infants successfully weaned differed only in developing a greater maximal inspiratory force (33.8 +/- 12.3 versus 23.3 +/- 15.0 cm H2O) and higher compliance (1.1 +/- 0.3 versus 0.7 +/- 0.3) during the preweaning treatment period. These results indicate that maturity and size play a significant role in the ability to wean a preterm infant from the ventilator successfully, that maximal inspiratory force and compliance are higher in preterm infants who can be successfully extubated, and that methylxanthines do not uniformly improve pulmonary function in all potentially extubatable preterm infants.