The room temperature powder pattern of abacavir hemisulfate (anti-HIV reverse transcriptase compound) was indexed using 2θ 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) Å, b=8.437(1) Å, c=14.259(2) Å, β=93.87(1)°. There are two formula units [(C14H16N6O)2.H2SO4] per unit cell and Dx=1.390 g∕cm3.