Coadministration of sofosbuvir (400 mg once daily) with atazanavir/ritonavir (300/100 mg once daily, with emtricitabine/tenofovir-DF 200/300 mg once daily) was studied in 8 patients coinfected with HIV and HCV. Emtricitabine and atazanavir PK parameters were unchanged. Tenofovir AUC and Cmin were unchanged, while Cmax increased by 40%. Ritonavir AUC and Cmax decreased by 21% and 32%, while Cmin was unchanged. Sofosbuvir AUC and Cmax increased by 342% and 109%, respectively. GS-331007 AUC and Ctau increased by 42% and 268%, respectively, while Cmax decreased by 23%. Although sofosbuvir and GS-331007 exposures are increased, these changes are not clinically significant. The authors conclude that sofosbuvir may be safely coadministered with atazanavir/ritonavir.