Several consistent medical problems have been encountered by astronauts during space flights. These include vestibular dysfunction, weight loss, increase in height, upward fluid shift, anemia, cardiovascular deconditioning, muscle atrophy, and bone loss. Almost all of these alterations can be attributed to the absence of gravitational force. Most are adaptive in nature and therefore reversible, but readaptation after returning to earth may cause further problems (e.g., in the case of vestibular dysfunction). The most recalcitrant and disturbing of all these problems is the relentless bone loss associated with negative calcium balance. This problem appears to be irreversible, and critical demineralization can occur after two years in a weightless state. Unless its mechanism is elucidated and preventive measures are taken, the bone loss may prove to be the medically limiting factor for the duration of space flight.