The purpose of this randomized controlled trial was to evaluate the analgesic efficacy of a series of applications of sulfurated mud baths in outpatients suffering from back pain. Within 2 weeks 13 patients took 6 sulfurated mud baths (group A) and 12 patients 6 tap water baths (group B) at home. Before the bath and over 48 h after starting the 1st and the 6th bath, pain intensity was evaluated by the patients according to a visual analogue scale (VAS). The main outcome parameter was the weighted (for time of observation) sum of pain intensity (SPI) after the 6th bath. The mean SPI in group A was 741 mm × h (95% CI 594–864 mm × h) and in group B 1,112 mm × h (95% CI 929–1,252 mm × h) (p = 0.009), suggesting a significantly stronger analgesic effect of a series of sulfurated mud baths than of a series of tap water baths.