In a controlled clinical trial, we allocated 48 subjects with acute low back pain but without neurological signs, at random to two treatment groups. The conservative treatments compared were passive mobilisation and manipulation of the lumbar spine and a regimen of microwave diathermy, isometric abdominal exercises and ergonomic instructions. The duration of low back pain symptoms was significantly shorter for subjects receiving mobilisation and manipulation; they also achieved symptom-free status with fewer treatment sessions. While the duration of symptoms before first treatment, the treatment administered, and the pretest forward flexion movement indices accounted for 44% of the variance in the duration of symptoms, a stepwise multiple regression analysis showed that treatment is the most significant factor in predicting the length of time before a subject achieves symptom-free status.