The room temperature powder pattern of abacavir hemisulfate (anti-HIV reverse transcriptase compound) was indexed using 2 theta values obtained from a powder pattern spiked with an internal standard. The resulting unit cell values for the monoclinic I2 cell [nonstandard setting of C2 (No. 5)] are a = 13.278(1) angstrom, b = 8.437(1) angstrom, c = 14.259(2) angstrom, beta = 93.87(1)degrees. There are two formula units [(C14H16N6O)(2).H2SO4] per unit cell and D-x = 1.390 g/cm(3). (c) 2005 International Centre for Diffraction Data.