A method of detecting a leak in an outer jacket of a metal halide lamp having an arc tube inside the outer jacket and operated by a ballast, where the ballast monitors an electrical characteristic of the lamp during start and detects a leak in the outer jacket when the electrical characteristic deviates from that of a corresponding metal halide lamp whose outer jacket does not have a leak. The electrical characteristic is one of six markers, namely (1) V'(E) at a predetermined E, where V'(E) is a derivative of lamp voltage V as a function of cumulative energy E delivered to the ballast since ignition, (2) a value of E at which V'(E) reaches a maximum, (3) V at a predetermined E, (4) a local maximum of V'(E) up to a predetermined E, (5) a global maximum of V'(E), and (6) E required to achieve a predetermined V.