Long term storage stable pemetrexed-containing liquid pharmaceutical compositions are disclosed. The compositions can include pemetrexed or pharmaceutically acceptable salts thereof; an antioxidant selected from lipoic acid, dihydrolipoic acid, methionine and mixtures thereof; a chelating agent selected from lactobionic acid, sodium citrate, tribasic and mixtures thereof; and a pharmaceutically acceptable fluid. The pH of the compositions is in a range of about 8 to about 9.5. The pemetrexed-containing compositions have less than about 5% total impurities, on a normalized peak area response (“PAR”) basis as determined by high performance liquid chromatography (“HPLC”) at a wavelength of 227 nm, after at least about 18 months of storage at a temperature of from about 5° C. to about 25° C. Methods of preparing the formulation as well as methods of treatment of pemetrexed-susceptible diseases using the same are also disclosed.