In earlier studies by our group it was shown that the urinary tetrahydroaldosterone excretion is a more reliable test for the diagnosis of primary aldosteronism than aldosterone-18-glucuronide (‘urinary aldosterone’). However, in several patients with primary aldosteronism even the tetrahydroaldosterone values remained in the normal range. As the possible cause for this observation, the role of intestinal bacteria was considered which may transform tetrahydroaldosterone into 21-deoxytetrahydroaldosterone. Using a 21-deoxytetrahydroaldosterone radioimmunoassay, this hypothesis could be confirmed. The sums of the urinary 21-deoxytetrahydroaldosterone and tetahydroaldosterone excretions revealed to be of better diagnostic value than the tetrahydroaldosterone values alone.