This prospective study investigated hypertension and renal vasoconstriction developing during the 1st year after renal transplantation in patients randomly allocated to treatment with FK 506 (n = 28) or CyA (n = 13). Starting doses were 0.2-0.3 mg/kg per day for FK 506 and 5-8 mg/kg per day for CyA: doses were subsequently adjusted to trough levels (5-15 ng/ml for FK 506 and 100-150 ng/ml for CyA). We compared 24-h ambulatory blood pressure measurement, antihypertensive treatment, serum creatinine, and resistance index (RI), measured by Doppler ultrasound at the level of the interlobar artery. Until month 2 of treatment, FK 506-treated patients had a significantly lower RI (8%) and better renal graft function, as evidenced by significantly lower serum creatinine values. Some 13% of FK 506-treated patients, compared to 70% of CyA-treated patients (P < 0.01), needed additional antihypertensive drugs after transplantation to keep blood pressure stable. FK 506 treatment, at the above-mentioned dosages, was associated with a significantly higher number of infections (urinary tract infection, pyelonephritis, and pneumonia). We conclude that CyA produces greater renal vasoconstriction and systemic hypertension than FK 506, as reflected in higher renal interlobar artery RI values and a greater need for antihypertensive treatment. After 2 months of treatment and a reduction in CyA trough levels, the renal effects (i.e., lower RI and lower creatinine values), but not the systemic hypertensive effects, disappear.