INTRODUCTION: The role of aminophylline in the treatment of severe acute asthma in the pediatric critical care unit (PCCU) is not clear. We sought to examine the association of aminophylline treatment with PCCU length of stay and time to symptom improvement. MATERIAL AND METHODS: Patients with severe acute asthma who were admitted to our PCCU and received aminophylline infusion were retrospectively compared with similar patients who did not receive aminophylline. The primary outcome measure was functional length of stay (i.e. time to which patients could be transferred to a general pediatric ward bed). A secondary outcome was time to symptom improvement. RESULTS: Adjusted functional length of stay was longer for subjects who received aminophylline (n = 49) than for the patients who did not (n = 47) (hazard ratio 0.396, p < 0.001), as well as the time for symptom improvement (hazard ratio 0.359, p < 0.001). In the group of subjects receiving aminophylline, those with a serum theophylline level >/= 10 mcg/ml (therapeutic) (n = 31) had longer functional length of stay (hazard ratio 0.457, p = 0.0225) and time to symptom improvement (hazard ratio 0.403, p = 0.0085) than those with levels < 10 mcg/ml (sub-therapeutic) (n = 18). CONCLUSIONS: The addition of aminophylline to therapy with corticosteroids and inhaled beta-agonists was associated with statistically and clinically significant increases in functional length of stay and time to symptom improvement in the PCCU. This potential morbidity supports the National Asthma Education and Prevention Program guideline proscribing aminophylline use in acute asthma.