Topical glucocorticosteroids are useful in the treatment of various skin diseases. Although many corticosteroids are available today, there is still a need for highly potent compounds with minimal adverse effects. Methylprednisolone aceponate (MPA) has recently been synthesized. Its activity has been evaluated using the vasoconstrictor assay and the poison ivy test (rhus dermatitis) in 19/20 healthy volunteers of either sex. Comparable blanching was found with MPA in a cream vehicle, in an ointment and a fatty ointment. Vasoconstriction and suppression of experimentally-induced poison ivy contact dermatitis were dose-dependent in the concentration range 0.01% to 0.5% MPA. Concentrations of MPA of at least 0.05% were significantly active. Following the highest dose, blanching was close to the maximum which can be obtained. This finding, and the improvement of rhus dermatitis, suggest that MPA belongs to the highly potent local glucocorticosteroids.