To determine the efficacy of theophylline treatment in infants at increased risk for SIDS, we obtained 24-hour cardiorespiratory recordings (pneumograms) in 80 infants given theophylline in whom the initial pneumogram was abnormal. Fifty-three infants had a clinical diagnosis of near-SIDS, and 27 were asymptomatic siblings with a positive family history for SIDS. The initial pneumogram was obtained at a mean age of 6.9 weeks, and the repeat pneumogram 2.3 weeks later, when the mean theophylline blood concentration was 11.2 +/- 0.5 micrograms/ml. Theophylline treatment resulted in comparable and highly significant improvements in both groups. Among all 80 infants, apnea density decreased from 1.6 +/- 0.2% (SEM) to 0.3 +/- 0.1% (P less than 0.001), periodic breathing episodes/100 minutes decreased from 2.7 +/- 0.4 to 0.3 +/- 0.1 (P less than 0.001), and the longest apneic period decreased from 13.5 +/- 0.7 to 10.1 +/- 0.5 seconds (P less than 0.001). Findings on the pneumogram became completely normal with theophylline therapy in 87% of infants with near-SIDS and 81.5% of asymptomatic siblings. Pneumogram normalization was associated with absence of further symptomatic sleep apnea in the near-SIDS group and with continued absence of any clinical symptoms in the asymptomatic family history group. There were no deaths from SIDS.