Abstract: An implantable sensor for monitoring the condition of an organ, such as the bladder, of a recipients' body. The sensor (100) comprises a carrier (102), such as a tubular housing that is attachable to the outer surface of the organ. An impedance-varying means (104), such as a saline solution, is carried by the carrier (102). The impedance-varying means (104) within the carrier varies as the condition of the organ changes to cause in output state of the sensor (100). The sensor (1000) can be used for the purposes of monitoring the filling of the bladder, making bladder volume information to the recipient or another person, and causing an alarm signal to be generated when a preset bladder volume is reached.