A DRAM refresh method used with a memory system organized into rows of memory cells, each of which has an associated data retention time, with the system arranged to refresh predefined blocks of memory cells simultaneously. For each block of memory cells that are to be refreshed simultaneously, the minimum data retention time for the memory cells in the block is determined. Then, an asymmetric refresh sequence is created which specifies the order in which the blocks of memory cells are refreshed, such that the blocks having the shortest minimum data retention times are refreshed more often than the blocks having longer minimum data retention times.