This 52-week study was designed to assess the safety and efficacy of fluticasone propionate/salmeterol combination (FSC) 250/50 micrograms versus fluticasone propionate (FP) 250 micrograms in subjects with persistent asthma symptomatic on open-label FP 100 micrograms. The primary objective of this study was to show that FSC 250/50 micrograms was superior to FP 250 micrograms at increasing pulmonary function as measured by forced expiratory volume in 1 second over a 52-week treatment period. A secondary objective was to compare the rate of asthma attacks defined as (1) a sustained 2-day decrease in morning peak expiratory flow or increase in albuterol use for 2 consecutive days, (2) an asthma exacerbation requiring systemic corticosteroids, or (3) an unscheduled clinic or hospital visit for acute asthma symptoms. Three hundred six subjects received FSC 250/50 micrograms and 315 subjects received FP 250 micrograms. Both treatments were administered twice daily. Treatment with FSC 250/50 micrograms resulted in a significant improvement in lung function compared with FP 250 micrograms (p < 0.001). Additionally, treatment with FSC 250/50 micrograms resulted in a reduction in the rate of exacerbations of asthma (i.e., requiring systemic corticosteroids or unscheduled urgent care intervention) compared with FP 250 micrograms (0.170 versus 0.273, respectively; p = 0.017). There was no differentiation between treatments for less severe attacks of asthma. FSC 250/50 micrograms showed consistently greater improvement in lung function, symptom control, and decreased albuterol use. In addition, FSC 250/50 micrograms-treated subjects experienced fewer severe asthma exacerbations than subjects treated with FP 250 micrograms.