OBJECTIVES: 1) To compare antihypertensive efficacy of Losartan and Irbesartan controlling for baseline Seated Diastolic and Systolic Blood Pressure (SeDBP&SeSBP) before treatments, and 2) to compare antihypertensive efficacy of Losartan and Irbesartan between gender controlling for 1) baseline SeDBP&SeSBP, and 2) age. METHODS: An experimental design was performed.  All hypertensive patients who were prescribed 50 mg. Losartan once a day or 150 mg. Irbesartan once a day for hypertension during January 1-June 30, 2008 were the population framework.  Exclusion criteria included concomitant diseases that would present safety hazards and concomitant medications that might interfere with the assessment of efficacy or safety e.g., drugs known to affect BP.  Simple random technique was employed.  The α 0.05, power 0.90 and effect size 0.07 were set to generate 200 samples in each group (total 400). The average baseline SeDBP&SeSBP of Losartan group and Irbesartan group were 91.86±13.73, 150.76±19.14 and 89.56±10.69, 148.42±15.45 respectively.  Baseline SeDBP&SeSBP were used as covariates.  After medications for eight weeks SeDBP&SeSBP were measured and compared. RESULTS: Total 400 (100%) patients, mostly 267 (66.80%) were female, 133 (33.33%) were male with average age 63.31±12.52 years.  After treatment the average SeDBP of Losartan or Irbesartan groups were 77.26±9.76 and 74.43±9.84 mm. Hg respectively (p=.000, ANCOVA).  After treatment the average SeSBP of Losartan or Irbesartan groups were 131.72±15.17 and 127.50±12.22 mm. Hg respectively (p=.010, ANCOVA).  When controlled age (covariate) and added gender (fixed factor) to the model, the means of SeDBP&SeSBP of Losartan group and Irbesartan group were 77.26±9.76, 128.81±12.89 and 72.43±9.84, 127.50±12.22 mm. Hg respectively (p=0.000 and 0.029, two way ANCOVA without gender interaction (p=0.520, 0.101). CONCLUSIONS: Irbesartan 150 mg. once a day could significantly lower seated systolic and diastolic blood pressure in hypertension patients better than Losartan 50 mg. once a day. Gender made no differences on efficacy of the two drugs.