The stability of theophylline supplied as a premixed injection and of methylprednisolone sodium succinate in admixtures containing both drugs was studied. Solutions containing theophylline in concentrations of 4.0 mg/mL and 0.4 mg/mL were used. Methylprednisolone sodium succinate was added to each solution to produce a final concentration of 0.5 mg/mL and 2.0 mg/mL of methylprednisolone alcohol, a pharmacologically active form of methylprednisolone sodium succinate. Each admixture was prepared in triplicate, and samples were kept at room temperature in glass containers. Immediately after admixture and at 3, 6, 12, and 24 hours, samples were visually inspected, tested for pH, filtered, and assayed in duplicate by high-performance liquid chromatography for theophylline concentration and for both methylprednisolone sodium succinate and methylprednisolone alcohol content. Control solutions containing only one of the two drugs were also tested. No visual changes were observed. The addition of theophylline in 5% dextrose injection to the methylprednisolone sodium succinate solutions resulted in decreased pH values for all solutions, which did not vary significantly throughout the study period. Theophylline concentrations did not change significantly compared with baseline. In solutions containing theophylline 0.4 mg/mL with either 2.0 or 0.5 mg/mL of methylprednisolone sodium succinate, less than 90% of the initial methylprednisolone sodium succinate concentrations remained at 24 hours. However, within three hours after admixture preparation, methylprednisolone alcohol was detected in those solutions in increasing concentrations. A commercial preparation of premixed theophylline in 5% dextrose injection in a concentration of 4 mg/mL or less can be mixed with methylprednisolone sodium succinate in a final concentration of 2 mg/mL or less and administered intravenously within 24 hours after mixing.