Background: There is little scientific evidence to support the current practice of using oral glucocorticosteroids and antibiotics to treat patients with chronic rhinosinusitis and nasal polyps. Objective: We evaluated the effects of oral glucocorticoids and doxycycline on symptoms and objective clinical and biological parameters in patients with chronic rhinosinusitis and nasal polyps. Methods: In a double-blind, placebo-controlled, multicenter trial, we randomly assigned 47 participants with bilateral nasal polyps to receive either methylprednisolone in decreasing doses (32-8 mg once daily), doxycycline (200 mg on the first day, followed by 100 mg once daily), or placebo for 20 days. Participants were followed for 12 weeks. Patients were assessed for nasal peak inspiratory flow and symptoms and by nasal endoscopy. Markers of inflammation such as eosinophilic cationic protein (ECP), IL-5, myeloperoxidase, matrix metalloproteinase 9, and IgE were measured in nasal secretions. Concentrations of eosinophils, ECP, and soluble IL-5 receptor α were measured in peripheral blood samples. Results: Methylprednisolone and doxycycline each significantly decreased nasal polyp size compared with placebo. The effect of methylprednisolone was maximal at week 3 and lasted until week 8, whereas the effect of doxycycline was moderate but present for 12 weeks. Methylprednisolone significantly reduced levels of ECP, IL-5, and IgE in nasal secretions, whereas doxycycline significantly reduced levels of myeloperoxidase, ECP, and matrix metalloproteinase 9 in nasal secretions. Conclusion: This is the first double-blind, placebo-controlled study to show a significant effect of oral methylprednisolone and doxycycline on size of nasal polyps, nasal symptoms, and mucosal and systemic markers of inflammation.