Background. Rehabilitation of patients with low back pain forms an important component of musculoskeletal physical therapist practice, yet treatment outcomes often are poorly measured. Objective. The study examined the methods used to evaluate treatment outcomes and factors influencing the use of outcome measures by New Zealand physical therapists. Design. This cross-sectional study used qualitative and quantitative methods for data collection. Methods. Two focus groups were conducted: one in a private practice (n = 6) and one in a public hospital (n = 6). A survey questionnaire was mailed to all private practices listed in a telecommunication database and to outpatient physical therapy departments at public hospitals (n = 579). Results. The mail survey achieved a 65% response rate and showed that physical therapists use improvements in person-specific functional activities as their main outcome measure. Only 40% of the respondents reported using back-related outcome measures. The statistically significant factors determining their use were having a master's degree and an increased level of knowledge of outcome measurement, but these factors explained only 22% of the variance in the logistic regression model. Lack of time, frequently mentioned as a reason for not using standardized outcome measures, did not reach statistical significance. Limitations. The data collected relate to the physical therapists' reported or perceived behavior, which may be different from reality. Conclusion. Physical therapists do not routinely use outcome measures in their clinical practice. A master's degree and increased knowledge were statistically significant factors supporting increased use of outcome measures. Further research is needed on how to convey to practitioners that the information they provide can be useful and can improve patient outcomes., Outcome measures are tools for measuring the outcomes of health care interventions over time. For physical therapists, important outcomes include changes in patient impairments, activity limitations, participation restrictions, and quality [...]