OBJECTIVES: The window of opportunity (WOO) hypothesis suggests a limited time frame to stop rheumatoid arthritis (RA). We hypothesised that a WOO could either be represented by a hyperbolic ('curved') decline in the chance to achieve the outcome sustained drug-free remission (sDFR) over time, after which achieving sDFR is not possible anymore, or by a more gradual linear decline approaching zero chance to achieve sDFR. METHODS: Patients with RA (symptom duration <2 years) were included from two randomised trials: BehandelStrategieën (BeSt), n=508 and Induction therapy with Methotrexate and Prednisone in Rheumatoid Or Very Early arthritic Disease (IMPROVED), n=479. Cox-regression was performed to assess the shape of the association between symptom duration and sDFR (Disease Activity Score<1.6, no disease-modifying anti-rheumatic drugs for ≥1 year) for patients starting slow-acting monotherapy (IMPROVED, BeSt) or fast-acting combination therapy (BeSt). Likelihood ratio tests were used to compare the fit of linear and non-linear models in both databases separately. Predictions from the best fitting models were used to assess whether the absolute risk to achieve sDFR approaches zero with increasing symptom duration. RESULTS: In BeSt and IMPROVED, 54/226 and 110/421 patients achieved sDFR with fast-acting treatment, and 53/243 (BeSt) with slow-acting treatment. Non-linear models did not fit better than linear models (fast-acting treatment BeSt p=0.743, IMPROVED p=0.337; slow-acting treatment BeSt p=0.609). After slow-acting monotherapy, linear models declined steeper. None of the models approached zero chance to achieve sDFR over time. CONCLUSIONS: The chance to achieve sDFR decreased gradually over time, and decreased fastest in patients starting slow-acting monotherapy. In both treatment groups, we found no evidence for a WOO within 2 years symptom duration.