Background: Asthma treatment guidelines recommend addition of controller medications for patients with poorly controlled asthma. We compared the effectiveness of once-daily oral controller therapy with either an antileukotriene receptor antagonist (montelukast) or low-dose theophylline added to existing medications in patients with poorly controlled asthma. Methods: We conducted a randomized, double-masked, placebo-controlled trial in 489 participants with poorly controlled asthma randomly assigned to placebo, theophylline (300 mg/d), or montelukast (110 mg/d). Participants were monitored for 24 wk to measure the rate of episodes of poor asthma control (EPACs) defined by decreased peak flow, increased P-agonist use, increased oral corticosteroid use, or unscheduled health care visits. Observations: There was no significant difference in EPAC rates (events/person/yr) Compared with placebo: low-dose theophylline, 4.9 (95% confidence interval [CI], 3.6-6.7; not significant); montelukast, 4.0 (95% CI, 3.0-5.4; not significant); and placebo, 4.9 (95% CI, 3.8-6.4). Both montelukast and theophylline caused small improvements in prebronchodilator FEV1 of borderline significance. Nausea was more common with theophylline only during the first 4 wk of treatment. Neither treatment improved asthma symptoms or quality of life. However, in patients not receiving inhaled corticosteroids, addition of low-dose theophylline significantly (p < 0.002) improved asthma control and symptoms as well as lung function. Conclusions: Neither montelukast nor low-dose theophylline lowered the EPAC rate of poor asthma control in patients with poorly controlled asthma despite improved lung function. For patients not using inhaled corticosteroids, low-dose theophylline improved asthma symptom control more than montelukast or placebo, and provides a safe and low-cost alternative asthma treatment.