Background: The motive behind conducting this study was to determine if better control of asthma can be achieved by adding a second controller medication and to assess its use to reduce the dose of inhaled steroids. Objectives: The study aimed to determine whether either oral sustained-release theophylline or montelukast added to inhaled steroids improved clinical symptoms and pulmonary function test parameters when compared to high-dose steroids alone. Methods: Ninety patients with incompletely controlled asthma were allocated, in a randomised, double-blind fashion, to one of three treatment groups: group A: double dose of inhaled budesonide (400 µg b.i.d.), group B: 400 mg oral sustained-release theophylline plus budesonide (200 µg b.i.d.) and group C: 10 mg montelukast plus budesonide (200 µg b.i.d.). The primary endpoints were forced expiratory volume in 1 s (FEV₁) and mean morning peak expiratory flow rate (PEFR). Results: All three groups had improved FEV₁ and PEFR at 8 weeks (p < 0.001). Group C increased their PEFR by 18.7 l/min (95% confidence interval, CI, 12.4–25.1) more than group A and by 19.8 l/min (95% CI 13.4–26.1) more than group B (both p = 0.001). Similarly, group C had a 114 ml (95% CI 45–183 ml) greater improvement in FEV₁ than group A and a 95 ml (95% CI 26–164 ml) greater improvement than group B (both p = 0.01). Conclusions: Addition of montelukast to budesonide is safe and results in greater improvement in pulmonary function test parameters than high-dose budesonide treatment or addition of theophylline.