Low-pressure mercury vapor discharge lamp has an at least partly substantially cylindrical discharge vessel (10) with a length Ldv and with an internal diameter Din. The discharge vessel (10) encloses, in a gastight manner, a discharge space (13) provided with a inert gas mixture and with mercury. The discharge vessel (10) comprises discharge means (electrodes 20a; 20b) for maintaining a discharge in the discharge space (13). According to the invention, the ratio of the weight of mercury mHg in the discharge vessel (10) to the product of the internal diameterDin and the length of the discharge vessel Ldv is given by the relation: wherein C (0.01 (g/mm2. Preferably, 0.0005 (C(0.005(g/mm2. Preferably, the discharge vessel (10) contains less than 0.1 mg mercury. The discharge lamp according to the invention operates under unsaturated mercury conditions.