This is an open label, single period study in 24 male DMD subjects consisting of children (ages 4-12, inclusive) and adolescents (ages 13-16, inclusive) with at least 12 subjects between the ages of 4-12 (children). Subjects taking maintenance corticosteroid therapy will be required to take their dose of corticosteroid 24 hours (± 2 hours) prior to the first dose of deflazacort on Day 1. Subjects receiving deflazacort as maintenance corticosteroid therapy must take their last dose at least 30 days prior to the first dose of deflazacort on Day 1. Concomitant corticosteroid therapy will be prohibited during the study while the subject is taking deflazacort. On Day 1, a single oral dose of deflazacort under fasting conditions will be administered in the CRU followed by blood sampling for plasma analysis of DFZ and 21-desacetyl-DFZ for 8 hours. Following the 8 hour PK sample on Day 1, subjects will be given medication to take at home for once-daily, morning dosing on Days 2 through 7 (+2 days). On Day 8 (+2 days), subjects will return for a PK sample predose and an 8th oral dose of deflazacort under fasting conditions which will be administered in the CRU followed by PK sampling for 8 hours.