The aim of this study was to investigate test-retest reliability of the Isernhagen Work System Functional Capacity Evaluation (IWS FCE) in a sample of patients (n = 30) suffering from Chronic Low Back Pain (CLBP) and selected for rehabilitation treatment. The IWS FCE consists of 28 tests that reflect work-related activities like lifting, carrying, bending, etc. In this study, a slightly modified IWS FCE was used. Patients were included in the study if they were still at work or were less than 1 year out of work because of CLBP. Participants' mean age was 40 years, the duration of low back pain ranged between 5 and 10 years. Fifteen patients (50%) were out of work for a mean of 17 weeks, and they all received financial compensation. Two FCE sessions were held with a 2-week interval in between. Means per session, 95% confidence intervals of the mean difference, one-way random Intra Class Correlations (ICC), limits of agreement, Cohen's kappa and percentage of absolute agreement were calculated where appropriate. An ICC of 0.75 or more, a kappa value of more than 0.60 and a percentage of absolute agreement of 80% were considered as an acceptable reliability. Tests of the IWC FCE were divided into tests with and tests without an acceptable test-retest reliability on the basis of the kappa values, the percentage of absolute agreement and the ICC values. Fifteen tests (79%) showed an acceptable test-retest reliability based on Kappa values and percentage of absolute agreement. Eleven tests (61%) showed an acceptable test-retest reliability based on ICC values.