This study reports on 105 patients with acute low-back pain given tizanidine (4 mg three times daily) plus ibuprofen (400 mg three times daily) or placebo plus ibuprofen (400 mg three times daily). Patients assessed their pain using visual analogue scales in a daily diary and the doctor assessed their condition at baseline and on days 3 and 7. Both groups were treated effectively, but earlier improvement occurred in patients given tizanidine/ibuprofen, particularly regarding pain at night and at rest. Doctors assessed the helpfulness of treatment: tizanidine/ibuprofen was significantly better than placebo/ibuprofen at day 3 (P = 0.05). Significant differences between treatments in favour of tizanidine/ibuprofen occurred in patients with moderate and severe pain at night (P less than 0.05), at rest (P less than 0.05) and those with moderate or severe sciatica (P less than 0.05). Significantly more patients given placebo/ibuprofen had gastro-intestinal side-effects compared with tizanidine/ibuprofen (P = 0.002). This supports previous work in animals showing that tizanidine mediates gastric mucosal protection against anti-inflammatory drugs. More patients given tizanidine/ibuprofen suffered drowsiness and other central nervous system effects (P = 0.025). In patients with severe acute low-back pain, however, some sedation and bed rest is advantageous. This study shows that tizanidine/ibuprofen is more effective in the treatment of moderate or severe acute low-back pain than placebo and ibuprofen alone.