The purpose of this study is to evaluate the safety and efficacy of clobazam as adjunctive therapy in the treatment of seizures which lead to drop attacks (drop seizures) in patients 2 to 60 years of age with Lennox-Gastaut Syndrome (LGS). Patients will be enrolled at approximately 65 sites in the U.S. and ex-US for up to 23 weeks. Patients will be randomly assigned to either a low, medium or high dose, or placebo. The study will include a baseline period, a titration period and a maintenance period. After the maintenance period, patients will either continue into an open-label extension study or enter the taper period with a final visit 1 week after the last dose.