This study will assess the efficacy of Botulinum Toxin A (BOTOX®) injected into the lumbar paravertebral muscles for reducing pain and disability in subjects suffering from sub-acute low back pain (duration 6 to 16 weeks) arising from an identifiable muscle strain injury or back trauma. The treatment modality and techniques used are based upon three successful prior open-labeled pilot studies done by this research group investigating the effect of BOTOX® on relief of chronic low back pain. This study, however, will employ a prospective, double-blind, randomized, placebo controlled trial to control for any placebo or mechanical trigger-point injection effects. Subjects will also be assessed for 6 months to define the duration of efficacy of BOTOX®. Sixty subjects will be randomly assigned to one of two arms. There will be two parts to this study. In the first both groups will receive either BOTOX® or placebo injection into the lumbar paravertebral muscles. The subjects will be assessed using validated scales for pain and disability prior to injection and weekly to monthly thereafter for three months. Subjects who improve will have a single follow-up visit at 6 months while those subjects who do not improve will enter the second, open-label part where they will receive BOTOX® injection and be monitored weekly to monthly for the next 3 months, giving a total subject participation period of 6 months. All subjects will continue to receive any medication or physiotherapy per standard of care but those treatments will be recorded and controlled for in the data analysis.