The paper by Kraemer et al1 demonstrated the potential for theophylline toxicity in association with influenza B. This article adds to the previous literature demonstrating increased serum theophylline concentrations during maintenance dosage in association with prolonged febrile illnesses.2,3 The authors recommend appropriately that a serum theophylline measurement be performed when symptoms compatible with theophylline toxicity are observed. I would add to that recommendation the precaution that a subsequent theophylline dose not be given in the presence of nausea, vomiting, or unusual headache even if a serum concentration is not yet known.