PURPOSE: For years, heat has been used for comfort and analgesia is recommended as a first-line therapy in many clinical guidelines. Yet, there are questions that remain about the actual effectiveness of heat for a condition as common as chronic low back pain, and factors such as time of onset, optimal temperature, and duration of effect. MATERIALS AND METHODS: A randomized double-blinded controlled trial was designed to compare the analgesic response to heat delivered via pulses at 45°C (experimental group, N=49) to steady heat at 37°C (control group, N=51) in subjects with longstanding low back pain. Treatment lasted 30 minutes with follow-up out to four hours. The hypothesis was that the experimental group would experience a higher degree of analgesia compared to the control group. Time of onset and duration of effect were also measured. RESULTS: Both groups were similar in average duration of pain (10.3 years). The primary outcome measure was pain reduction at 30 minutes after the end of treatment, using a 10-points numeric pain scale. Reduction in pain was greater for the experimental group than the control group (difference in mean reduction = 0.72, 95% CI 0.15-1.29, p = 0.014). Statistically significant differences in pain levels were observed from the first measure at 5 minutes of treatment through 120 minutes after completion of treatment. Reduction in pain associated movement was greater in the active heat group than the placebo group (p = 0.04). CONCLUSION: High-level pulsed heat (45°C) produced significantly more analgesia as compared to steady heat at 37°C at the primary end point and for an additional 2 hours after treatment. The onset of analgesia was rapid, <5 minutes of treatment. The results of this trial provide insight into the mechanisms and properties of thermal analgesia that are not well understood in a chronic low back pain model.