A method is provided for adjusting a read voltage in a flash memory device. The method includes storing first program count information when first pages of flash memory cells are programmed, the first program count information indicating a number of bits having a first logic value from among bits of data programmed in the first pages of the flash memory cells, and obtaining first read count information by counting a number of bits having the first logic value from among bits of data read from the first pages of the flash memory cells, while reading data from the flash memory cells using read voltages. The read voltages are adjusted based on the difference between the first read count information and the first program count information.