The objectives of the study are to test the hypotheses that psychological distress affects subsequent low-back pain, and pain affects subsequent distress. Six hundred eighty-one participants in a randomized clinical trial of low-back pain treatments were followed for 18 months with assessments for pain, disability, and psychological distress at 6 weeks and 6, 12, and 18 months. Multivariable logistic regression modeling with generalized estimating equations was used to estimate effects. Current pain and disability increased the odds of subsequent psychological distress [pain: adjusted odds ratio (OR)=1.36, 95% confidence interval (CI)=1.07, 1.72; disability: adjusted OR=1.23, 95% CI=0.98, 1.55], and current distress increased the odds of subsequent pain and disability (pain: adjusted OR=1.51, 95% CI=1.24, 1.86; disability: adjusted OR=1.49; 95% CI=1.20, 1.85). Cross-sectional associations were much stronger than the longitudinal associations, suggesting bias in the former due to selection factors and/or temporal ambiguity. The longitudinal findings suggest that pain/disability and psychological distress may be causes and consequences of each other, although the associations are small.