Background: We aimed to investigate whether the addition of non-steroidal anti-inflammatory drugs or spinal manipulative therapy, or both, would result in faster recovery for patients with acute low back pain receiving recommended first-line care. Methods: 240 patients with acute low back pain who had seen their general practitioner and had been given advice and paracetamol were randomly allocated to one of four groups in our community-based study: diclofenac 50 mg twice daily and placebo manipulative therapy (n=60); spinal manipulative therapy and placebo drug (n=60); diclofenac 50 mg twice daily and spinal manipulative therapy (n=60); or double placebo (n=60). The primary outcome was days to recovery from pain assessed by survival curves (log-rank test) in an intention-to-treat analysis. This trial was registered with the Australian Clinical Trials Registry, ACTRN012605000036617. Findings: Neither diclofenac nor spinal manipulative therapy appreciably reduced the number of days until recovery compared with placebo drug or placebo manipulative therapy (diclofenac hazard ratio 1.09, 95% CI 0.84-1.42, p=0.516; spinal manipulative therapy hazard ratio 1.01, 95% CI 0.77-1.31, p=0.955). 237 patients (99%) either recovered or were censored 12 weeks after randomisation. 22 patients had possible adverse reactions including gastrointestinal disturbances, dizziness, and heart palpitations. Half of these patients were in the active diclofenac group, the other half were taking placebo. One patient taking active diclofenac had a suspected hypersensitivity reaction and ceased treatment. Interpretation: Patients with acute low back pain receiving recommended first-line care do not recover more quickly with the addition of diclofenac or spinal manipulative therapy.