The aim of the present study was to explore, for the first time, sex differences among patients diagnosed with stress-related pain before and after flotation restricted environmental stimulation technique (REST) treatment, delivered 12 times during seven weeks. The present study included 88 patients (69 women, 19 men) from three different studies (post hoc analysis). They had been diagnosed by a physician as having chronic stress-related muscle tension pain. The analyses indicated that the flotation-REST treatment had beneficial effects on stress, anxiety, depression, sleep quality and pain and that there were few sex differences. Women were more depressed than men before treatment, but after treatment there was no difference between sexes. However, there was a sex difference in the ability to endure experimentally induced pain, suggesting that men exhibited greater endurance both before and after the flotation-REST treatment. The results also showed, for the first time, that both sexes improved their ability to endure experimentally induced pain (higher scores for upper pain threshold) following the successful flotation-REST pain treatment.