An association has been reported between chronic infection with Chlamydia pneumoniae and the severity of asthma, and uncontrolled observations have suggested that treatment with antibiotics active against C. pneumoniae leads to an improvement in asthma control. We studied the effect of roxithromycin in subjects with asthma and immunoglobulin G (IgG) antibodies to C. pneumoniae > or = 1:64 and/or IgA antibodies > or = 1:16. A total of 232 subjects, from Australia, New Zealand, Italy, or Argentina, were randomized to 6 wk of treatment with roxithromycin 150 mg twice a day or placebo. At the end of 6 wk, the increase from baseline in evening peak expiratory flow (PEF) was 15 L/min with roxithromycin and 3 L/min with placebo (p = 0.02). With morning PEF, the increase was 14 L/min with roxithromycin and 8 L/min with placebo (NS). In the Australasian population, the increase in morning PEF was 18 L/min and 4 L/min, respectively (p = 0.04). At 3 mo and 6 mo after the end of treatment, differences between the two groups were smaller and not significant. Six weeks of treatment with roxithromycin led to improvements in asthma control but the benefit was not sustained. Further studies are necessary to determine whether the lack of sustained benefit is due to failure to eradicate C. pneumoniae.