To evaluate the effectiveness of theophylline on density of central apnoeas and on incidence of periodic breathing; to compare the effect with age-related resolution of pneumogram abnormalities. Ninety-six infants (mean age 27.4 days; mean birth weight 2502 gm) with abnormal pneumographic results were randomly allocated to receive theophylline or placebo. The control group consisted of 94 healthy age- and weight-matched infants. After four weeks of treatment infants in the theophylline group had a significantly lower incidence (2.1% vs. 4.3%) and density of apnoeas > or = 15 sec (0.001 vs. 0.003). There were significantly less short apnoeas (> or = 6 sec.) in theophylline treated than in control infants (p < 0.01). The number of periodic breathing episodes showed a significant, presumably age-related reduction in both treatment groups. We conclude that theophylline is an effective agent in reducing the incidence and duration of apnoeas beyond what would be expected from age-related maturation of breathing.