Despite the continuous development of surgical techniques and implants, a substantial number of patients still undergo surgery for chronic low back pain (CLBP) without any benefit, or even become worse. With the aim of finding predictors of functional and work status outcome, 264 patients with severe CLBP of long duration, randomised to surgical or non-surgical treatment, were characterized by socio-demographic, clinical, radiological and psychological variables. The variables were estimated as predictors of outcome at the 2-year follow-up. Univariate and multiple logistic regression analyses were used in both treatment groups. We found that a personality characterized by low neuroticism and low disc height were significant predictors of functional improvement after surgical treatment. Depressive symptoms predicted functional improvement after non-surgical treatment. Work resumption was predicted by low age and short sick leave in the surgical group, and by short sick leave in the non-surgical group. We conclude that improved selection of successful surgical candidates with CLBP seems to be promoted by attention to severe disc degeneration, evaluation of personality traits and shortening of preoperative sick leave.