Objective: The optimal time to start biologics in polyarticular juvenile idiopathic arthritis (JIA) remains uncertain. The Childhood Arthritis and Rheumatology Research Alliance (CARRA) developed 3 consensus treatment plans (CTPs) for untreated polyarticular JIA to compare strategies for starting biologics. Methods: Start Time Optimization of Biologics in Polyarticular JIA (STOP-JIA) was a prospective, observational, CARRA Registry study comparing the effectiveness of 3 CTPs: 1) the step-up plan (initial nonbiologic disease-modifying antirheumatic drug [DMARD] monotherapy, adding a biologic if needed, 2) the early combination plan (DMARD and biologic started together), and 3) the biologic first plan (biologic monotherapy). The primary outcome measure was clinically inactive disease according to the provisional American College of Rheumatology (ACR) criteria, without glucocorticoids, at 12 months. Secondary outcome measures included Patient-Reported Outcomes Measurement Information System (PROMIS) pain interference and mobility scores, inactive disease as defined by the clinical Juvenile Arthritis Disease Activity Score in 10 joints (JADAS-10), and the ACR Pediatric 70 criteria (Pedi 70). Results: Of 400 patients enrolled, 257 (64%) began the step-up plan, 100 (25%) the early combination plan, and 43 (11%) the biologic first plan. After propensity score weighting and multiple imputation, clinically inactive disease according to the ACR criteria was achieved in 37% of those on the early combination plan, 32% on the step-up plan, and 24% on the biologic first plan (P = 0.17). Inactive disease according to the clinical JADAS-10 (score ≤2.5) was also achieved in more patients on the early combination plan than the step-up plan (59% versus 43%; P = 0.03), as was ACR Pedi 70 (81% versus 62%; P = 0.008), but generalizability was limited by missing data. PROMIS measures improved in all groups, but without significant differences. Twenty serious adverse events were reported (mostly infections). Conclusion: Achievement of clinically inactive disease without glucocorticoids did not significantly differ between groups at 12 months. While there was a significantly higher likelihood of early combination therapy achieving inactive disease according to the clinical JADAS-10 and ACR Pedi 70, these results require further exploration.