The results of a prospective study of 134 patients with lower back pain suggest that nonorganic factors are better predictors of return to work than organic findings. Patients who returned to work had fewer job, personal, or family related problems. There were no significant differences between patients who returned to work and those who did not when comparing myelograms, computed tomographic scans, or roentgenographs. The only significant difference in physical organic findings was for muscle atrophy. Patients who did not return to work had a statistically higher incidence rate of muscle atrophy. Length of time off from work was significantly related to outcome, but when patients were categorized according to time off the job, different factors predicted failure to return for patients off work for less than 6 months and patients off for more than 6 months. For patients off for less than 6 months, important predictors were a high Oswestry score, history of leg pain, family relocation, short tenure on the job, verbal magnification of pain, reports of moderate to severe pain on superficial palpation, and positive reaction to a "sham" sciatic tension test. None of these was a significant predictor for the group off for more than 6 months. For the group off work for more than 6 months, previous injuries, and stability of family living arrangements were among the significant predictors not significant for the group off less than 6 months. Using 21 factors selected from a larger group of 92 factors, three statistically significant (P less than or equal to 0.001) predictive measures were developed. These measures predicted return to work for the total sample, and for the two subgroups (off more than, or less than 6 months) more accurately than did the total set of 92 factors.