An apparatus for thermal coagulation and more particularly to a system for applying heat to the endometrium of a human uterus of the type comprising a heated balloon catheter having a reciprocating piston for improved circulation of fluid within a distensible bladder. The present invention includes a heated balloon catheter having a fluid delivery tube connected to a distensible bladder. The catheter further includes a reciprocating piston which moves along a linear path within the distensible bladder to cause vigorous fluid agitation therein. The agitation caused by the piston efficiently mixes the fluid within the balloon without requiring hot fluid to circulate through the fluid delivery tube.