A clinical prediction rule to identify patients most likely to respond to spinal manipulation has been published and widely cited but requires further testing for external validity. We performed a pre-planned secondary analysis of a randomised controlled trial investigating the efficacy of spinal manipulative therapy in 239 patients presenting to general practice clinics for acute, non-specific, low back pain. Patients were randomised to receive spinal manipulative therapy or placebo 2 to 3 times per week for up to 4 weeks. All patients received general practitioner care (advice and paracetamol). Outcomes were pain and disability measured at 1, 2, 4 and 12 weeks. Status on the clinical prediction rule was measured at baseline. The clinical prediction rule performed no better than chance in identifying patients with acute, non-specific low back pain most likely to respond to spinal manipulative therapy (pain P = 0.805, disability P = 0.600). At 1-week follow-up, the mean difference in effect of spinal manipulative therapy compared to placebo in patients who were rule positive rather than rule negative was 0.3 points less on a 10-point pain scale (95% CI -0.8 to 1.4). The clinical prediction rule proposed by Childs et al. did not generalise to patients presenting to primary care with acute low back pain who received a course of spinal manipulative therapy.