We report herein the case of a premature infant who presented with failure to thrive, hyponatremia, hyperkalemia and metabolic acidosis. Initial serum hormone profiling suggested isolated hypoaldosteronism (aldosterone: 0.01 pg/ml, normal range: 50-900 pg/ml). A gas chromatography-mass spectrometry spot urinary steroid profile showed grossly elevated levels of 18-hydroxy-tetrahydro- 11-dehydrocorticosterone (18-hydroxy-THA: 5,893 μg/l; normal upper limit 36 μg/l) and tetrahydroaldosterone (TH-Aldo: 5,749 μg/l; normal upper limit 36 μg/l) which are aldosterone precursor metabolite and aldosterone metabolite, respectively. Thus, aldosterone synthase deficiency was excluded and pseudohypoaldosteronism (PHA) was suggested. A repeated test after dilution of the serum revealed a very high level of aldosterone (6,490 pg/ml), confirming the diagnosis of PHA in this case. © 2010 S. Karger AG.