Background: The Tampa Scale for Kinesiophobia (TSK) and the Fear-Avoidance Beliefs Questionnaire (FABQ) are frequently used questionnaires for pain-related fear, and the visual analogue scale (VAS) is for pain. Purpose: This study aims to determine the smallest detectable change (SDC) of these questionnaires in patients with acute low back pain (LBP), as well as ceiling and/or floor effects. Results: The SDC for the TSK (scoring range: 17-68) was 9.2 (95% CI: 8.4; 10.3). The estimated SDC was 9.4 (95% CI: 8.5; 10.6) and 12.7 (95% CI: 11.5; 14.1) for the FABQ physical activity subscale (scoring range: 0-24) and/or work subscale (scoring range: 0-42), respectively. For the VAS for pain (0-100 mm), the SDC turned out to be 36.2 mm (95% CI: 32.4; 41.0). The FABQ physical activity subscale and pain (VAS) seem to have considerable problems in detecting improvement and deterioration. The TSK appears to be able to detect improvement as well as deterioration. Conclusion: The SDCs of the (subscales of) questionnaires range from 18% to 40%. Floor and/or ceiling effects were detected for most scales, except the TSK total and the TSK activity avoidance subscale. These results should be considered when using these questionnaires as measures of therapeutic change in acute LBP.