OBJECTIVE: The aim of this study was to assess the clinical effect of antibiotic treatment in a cohort of patients with low back pain (LBP) and Modic changes Type 1 (bone oedema) following a lumbar herniated disc. DESIGN: This was a prospective uncontrolled trial of 32 LBP patients who had Modic changes and were treated with Amoxicillin-clavulanate (500 mg/125 mg) 3 x day for 90 days. All patients had previously participated in a randomized controlled trial (RCT) that investigated active conservative treatment for a lumbar herniated disc (n=166). All patients in that RCT who had Modic changes and LBP at 14 months follow-up (n=37) were invited to participate in this subsequent antibiotic trial but five did not meet the inclusion criteria. RESULTS: Twenty-nine patients completed the treatment, as three patients dropped out due to severe diarrhoea. At the end of treatment and at long-term follow-up (mean 10.8 months) there was both a clinically important and statistically significant (p< 0.001) improvement in all OUTCOME MEASURES: LBP intensity, number of days with pain, disease- and patient-specific function, and global perceived effect. CONCLUSIONS: In this uncontrolled trial, the clinical effect of antibiotic treatment was large in a group of patients with Modic changes suffering from persistent LBP following a disc herniation. These results provide tentative support for a hypothesis that bacterial infection may play a role in LBP with Modic changes and indicate the need for randomised controlled trials to test this hypothesis.