OBJECTIVE: To determine which literature-based definitions of low back pain (LBP) could be combined to produce sufficiently similar sets for use in a meta-analysis. STUDY DESIGN AND SETTING: A group of six international experts participated in an e-mail-administered Delphi process. Literature-based LBP definitions were preliminarily classified into 14 sets within four outcome types: pathology, symptoms and care-seeking, functional limitations, and participation. Experts independently rated their level of agreement that each outcome definition belonged in its assigned set using a seven-point Likert scale. After each round, results were synthesized and revised classifications were fed back to the experts who were asked to consider them before rerating the outcome definitions. RESULTS: The experts completed three Delphi rounds and reached consensus on the categorization of 115/119 (97%) of the outcome definitions. There were 20 final sets of outcomes identified: three sets of pathology outcomes, two sets each of functional limitation and participation outcomes, and 13 sets of symptom and care-seeking outcomes. CONCLUSIONS: In a research area that currently lacks uniformly accepted definitions of outcomes, we successfully used a Delphi consensus process to reach substantial agreement on combinable LBP outcomes that would be combinable for a meta-analysis.