This study aims to determine the effect of grapefruit juice (GJ) on microemulsion cyclosporine (CsA) in 11 African American subjects, and it was compared to those in 11 Caucasian subjects. Each subject received two oral doses of CsA with water (W) or GI as well as i.v. CsA. Regardless of race, GJ significantly increased the peak concentration (Cmax) and area under the time-curve (AUC) of CsA; however, the magnitude of GJ effects was different between African American subjects and Caucasian subjects (p = 0.0003). GJ increased peak concentration of CsA by 39% in African American subjects, while the difference in Caucasian subjects was only 8% (p > 0.05). GJ also increased AUC of CsA in African American subjects by 60%, while GJ increased that in Caucasian subjects by 44% (p = 0.0001). The absolute bioavailability of CsA was 21% lower in African American subjects compared with Caucasian subjects when it was given with water (p = 0.048), but these differences disappeared when it was given with GJ (p = 0.6). These findings suggest that concurrent administration of GJ increases the bioavailability of CsA in African American subjects in greater magnitude compared with Caucasian subjects.