Diagnosis of primary aldosteronism should be easy in patients with unprovoked hypokaliemia and hypertension. However, it is more difficult to differentiate patients with primary normokaliemic aldosteronism from those with essential hypertension with low plasma renin activity. The aim of this study was to evaluate the appropriate testing of diagnosis and differential diagnosis of primary aldosteronism. The study group consisted of 17 patients with primary aldosteronism (12 with aldosteronoma and 5 with bilateral hyperplasia) and 20 patients with essential hypertension. The finding of increased serum aldosterone level using a single measurement, without anti-hypertensive therapy and on normal sodium intake is sufficient to confirm the diagnosis of primary aldosteronism with specificity of 95% and sensitivity of 94%. Upright posture test is shown to be fairly good tool in differentiation aldosteronoma and bilateral adrenal hyperplasia. Specificity of this test is 100% and sensitivity 88%. Although primary aldosteronism is not a common cause of hypertension it is reasonable to screen for this disorder because complete cure can be provided in a large percentage of patients.