The U.S. has not allowed gay men to donate blood since the 1980s - when HIV and AIDS cases grew, because gay men had a higher rate of infection than the general population.
Wisconsin Senator Tammy Baldwin is one of 85 Democratic members of Congress who have sent a letter to U.S. Health and Human Services Secretary Kathleen Sebelius, urging her to end the ban.
The letter says the ban promotes discrimination, based on sexual orientation.
People with AIDS/HIV can transmit the infection through sexual activity or an exchange of certain bodily fluids and by sharing needles.
The disease can also be passed along through exposure to infected blood. The U.S. has enacted rigorous procedures for testing blood supplies, before they're given to patients.