OBJECTIVES: To determine whether greater effusion-synovitis volume and infrapatellar fat pad (IFP) signal intensity alteration differentiate incident accelerated knee OA (KOA) from a gradual onset of KOA or no KOA. METHODS: We classified three sex-matched groups of participants in the Osteoarthritis Initiative who had a knee with no radiographic KOA at baseline (recruited 2004-06; Kellgren-Lawrence <2; n = 125/group): accelerated KOA: ⩾1 knee progressed to Kellgren-Lawrence grade ⩾3 within 48 months; common KOA: ⩾1 knee increased in radiographic scoring within 48 months; and no KOA: both knees had the same Kellgren-Lawrence grade at baseline and 48 months. The observation period included up to 2 years before and after when the group criteria were met. Two musculoskeletal radiologists reported presence of IFP signal intensity alteration and independent readers used a semi-automated method to segment effusion-synovitis volume. We used generalized linear mixed models with group and time as independent variables, as well as testing a group-by-time interaction. RESULTS: Starting at 2 years before disease onset, adults who developed accelerated KOA had greater effusion-synovitis volume than their peers (accelerated KOA: 11.94 ± 0.90 cm3, KOA: 8.29 ± 1.19 cm3, no KOA: 8.14 ± 0.90 cm3) and have greater odds of having IFP signal intensity alteration than those with no KOA (odds ratio = 2.07, 95% CI = 1.14-3.78). Starting at 1 year prior to disease onset, those with accelerated KOA have greater than twice the odds of having IFP signal intensity alteration than those with common KOA. CONCLUSION: People with IFP signal intensity alteration and/or greater effusion-synovitis volume in the absence of radiographic KOA may be at high risk for accelerated KOA, which may be characterized by local inflammation.