Modic type 1 changes/bone edema in the vertebrae are present in 6 % of the general population and 35-40 % of the low back pain population. It is strongly associated with low back pain. The aim was to test the efficacy of antibiotic treatment in patients with chronic low back pain (> 6 months) and Modic type 1 changes (bone edema). The study was a double-blind RCT with 162 patients whose only known illness was chronic LBP of greater than 6 months duration occurring after a previous disc herniation and who also had bone edema demonstrated as Modic type 1 changes in the vertebrae adjacent to the previous herniation. Patients were randomized to either 100 days of antibiotic treatment (Bioclavid) or placebo and were blindly evaluated at baseline, end of treatment and at 1-year follow-up. Primary outcome, disease-specific disability, lumbar pain. Secondary outcome leg pain, number of hours with pain last 4 weeks, global perceived health, EQ-5D thermometer, days with sick leave, bothersomeness, constant pain, magnetic resonance image (MRI). 144 of the 162 original patients were evaluated at 1-year follow-up. The two groups were similar at baseline. The antibiotic group improved highly statistically significantly on all outcome measures and improvement continued from 100 days follow-up until 1-year follow-up. At baseline, 100 days follow-up, 1-year follow-up the disease-specific disability-RMDQ changed: antibiotic 15, 11, 5.7; placebo 15, 14, 14. Leg pain: antibiotics 5.3, 3.0, 1.4; placebo 4.0, 4.3, 4.3. Lumbar pain: antibiotics 6.7, 5.0, 3.7; placebo 6.3, 6.3, 6.3. For the outcome measures, where a clinically important effect size was defined, improvements exceeded the thresholds, and a trend towards a dose-response relationship with double dose antibiotics being more efficacious. The antibiotic protocol in this study was significantly more effective for this group of patients (CLBP associated with Modic I) than placebo in all the primary and secondary outcomes.