We report that HIV-1 infection of human cells in vitro leads to significant decreases in the intracellular concentration of NAD. This decrease varies with viral load and HIV strain. In tissue culture, cells lacking CD4 receptors or cells incubated with heat inactivated virus do not demonstrate this decrease in NAD. Nicotinamide, the amide form of the vitamin niacin, increases intracellular NAD levels in uninfected cells as expected. Our data demonstrate that nicotinamide also maintains increased intracellular NAD concentrations in HIV infected cells. We conclude that HIV induces a state of intracellular pellagra which is reversed by the administration of nicotinamide.