Gitelman's syndrome (GS) is a variant of Bartter's syndrome (BS) characterized by hypokalemic alkalosis, hypomagnesemia, hypocalciuria and secondary aldosteronism without hypertension. A 31-year-old Japanese man who had suffered from mild hypokalemia for 10 years was admitted to our hospital. He had metabolic alkalosis, hypokalemia and hypocalciuria. Since he had two missense mutations (R261C and L623P) in the thiazide-sensitive Na-Cl cotransporter (TSC) gene (SLC12A3), he was diagnosed as having GS. He showed hyperreninism and a high angiotensin I (Ang I) level, whereas his angiotensin II (Ang II) and aldosterone levels were not elevated. His angiotensin converting enzyme (ACE) activities were normal, and administration of captopril inhibited the production of Ang II and aldosterone. We evaluated the Ang II-forming activity (AIIFA) of other enzymes in his lymphocytes. Interestingly, chymase-dependent AIIFA was not detected in the lymphocytes. Together, these results suggest that the lack of chymase activity resulted in the manifestation of GS without hyperaldosteronism.