RATIONALE: Exposure in vivo therapy aims to reduce pain-related fear - a key maintaining factor of chronic low back pain- while increasing level of daily functioning, despite the pain. This is done by exposing patients to their most feared activities/movements, while behavioral experiments are performed that serve to correct catastrophic (erroneous) beliefs about pain. Yet, performing exposure exercises might be very threatening for patients and might encourage them to build in subtle safety-seeking behavior. Whether safety-seeking behavior should be allowed or not during therapy is heavily debated. Whereas some argue that it will only interfere with therapeutic progress because it prevents the disconfirming experience exposure tries to offer, other argue that it will facilitate therapeutic progress because it enhances one's sense of control, if used judiciously. So far (clinical-)experimental studies have provided mixed evidence nor have they lead to any clinical recommendation. Hence, in a replicated single-case experiment, we will compare exposure therapy with versus exposure without safety-seeking behavior versus exposure only.