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The hydrothermal evolution of the Sengan geothermal area, Northern Honshu, was studied using fluid inclusion microthermometry. Sengan is one of the most active geothermal areas in Japan, and it is dominated by andesitic to dacitic volcanic rocks of Pliocene and Quaternary age. Fluid inclusions were studied in hydrothermal minerals (quartz, calcite, anhydrite, wairakite) and in fractured igneous quartz phenocrysts from core samples retrieved from five geothermal exploration wells, which penetrated Quaternary and Tertiary formations, and intrusive rocks in or around the calderas. A caldera-hosted hydrothermal system was heated by a shallow intrusion, which produced hypersaline fluids. During the early stages some heterogeneity in fluid composition occurred, but during peak activity of the hydrothermal system, the fluid was dominantly low-salinity, and most likely of meteoric origin. We have reconstructed, by means of fluid inclusion studies, the structure of an extinct hydrothermal system in a Pliocene caldera filled with ignimbrites, and of an active hydrothermal system which is now being heated under the northern slope of Yakeyama volcano. Based on inclusion data, the aqueous fluids that circulated in the hydrothermal system ranged from very low saline (0–7 wt.% NaCl equiv.) to hypersaline (up to 60 wt.% NaCl equiv.), with temperatures ranging from 130 to 400 °C. [ABSTRACT FROM AUTHOR]