Angiotensin converting enzyme inhibitors (ACEI) and theophylline have been reported to decrease the elevated hemoglobin (Hgb) and hematocrit (Hct) levels in the renal transplant recipients with erythrocytosis. We conducted a prospective randomized, open labeled, crossover trial with theophylline, and an ACEI, fosinopril in nine stable renal transplant recipients with erythrocytosis. Aim of the study was to determine and compare the efficacy of these medications in stable renal transplant patients. At three months, compared to baseline, fosinopril significantly reduced the elevated hemoglobin (Hgb 17.2 +/- 0.6 vs. 14.9 +/- 1.4 gm/dL, p = 0.0023), and hematocrit levels (Hct 51.3 +/- 2.4 vs. 43.7 +/- 4.6%, p = 0.003). In contrast theophylline therapy was associated with a non-significant rise in hemoglobin (17.4 +/- 0.7 vs. 18.1 +/- 0.9gm/dL, p > 0.05) and hematocrit (52.4 +/- 2.7 vs. 54.7 +/- 3.9%, p > 0.05). With fosinopril compared to theophylline, there was a significant difference in the change in hemoglobin (baseline to three months 2.8 +/- 1.7 vs. -0.7 +/- 0.69 gm/dL respectively, p = 0.017), and the change in hematocrit (baseline to three months 9 +/- 6 vs. -2.3 +/- 2.7% respectively, p = 0.027). Four patients (44.4%) did not tolerate theophylline and did not complete the theophylline arm. To conclude, in our study, fosinopril effectively decreased the elevated hemoglobin and hematocrit in patients with post transplant erythrocytosis, and was superior to theophylline, while theophylline was ineffective and poorly tolerated in this condition.