The potential role of sonography in evaluating the response to therapy of persistent knee joint synovitis (KJS) was assessed in a longitudinal study in pre-and post-arthroscopic (AS) synovectomy in rheumatoid and psoriatic patients. At entry to the study ultrasound (US) detection of synovial proliferation was compared with arthroscopic visualization as the 'gold standard' reference. US joint effusion and synovial thickness measures and predominant patterns of synovial proliferation were recorded by comparing clinical and US indices before and at 2, 6 and 12 months after AS synovectomy, or after KJS relapse up to 24 months. A 12 month survival analysis of clinical and US outcomes of arthroscopic synovectomy was also performed. US detection of morphology and degree of synovial proliferation was correlated with AS macroscopic evaluation. After AS synovectomy, the clinical index and both US joint effusion and synovial thickness were significantly reduced, whereas US patterns of synovial proliferation did not show significant changes. US and clinical indices were significantly correlated in all follow-up measurements and US joint effusion was significantly increased in the relapsed compared with the non-relapsed KJS group. The probability at 12 months of reaching maximum improvement in US joint effusion and synovial thickness outcomes was 99 and 58%, respectively; that for clinical remission of KJS was 72%. Ultrasound evaluation has proven reliable and accurate by the arthroscopic gold standard in detecting changes of rheumatoid arthritis and psoriatic arthritis knee joint synovitis. The correlation of US with clinical findings in pre-and post synovectomy patients suggests that sonography can be used as an objective method in monitoring the response to therapy of inflammatory knee joint disease.