The hypothesis that increased lumbar sagittal mobility constitutes a risk factor for recurrent low back trouble was tested in respect of two specific groups of subjects thought likely to display this phenomenon: young females reporting recurrent back pain, and middle-aged adults reporting recurrent or persistent sciatic symptoms. Lumbar flexibility was estimated from back surface curvatures by a validated technique which uses a flexicurve to record mobility in upper (T12-L4) and lower (L4-S2) lumbar regions. Mean mobility for young females (<40 years) with recurrent back pain was less, though not significantly so, than those without back trouble. A loss of mobility was also found in young males with recurrent trouble. However, it was a proportionally greater loss than that found in the females and was statistically significant. The mean mobility for middle-age adults with recurrent sciatic symptoms was significantly reduced compared to those without a history of back trouble. Whilst these results failed to support the general hypothesis, it remains possible that aspects of increased mobility not studied here, such as localized segmental instability or abnormal coupled motions, may be associated with an increased frequency of recurrent low back trouble.