A long-term prospective study was performed to evaluate the safety and long-term outcome of surgical arthroscopy (AS) for persistent rheumatoid (RA) and psoriatic (PsA) knee joint synovitis (KJS). Local signs of joint inflammation (tenderness, swelling, "ballottement') and range of motion (ROM) were scored and the sum, taken as a global outcome measure, was recorded in 17 RA and 18 PsA knees, both before and at follow-up periods of 2, 6, 12, 24 and 36 months after surgical AS (knee joint synovectomy; meniscal curettage, cartilage shaving or chondrectomy, according to the degree of cartilage damage). A survival analysis (Kaplan-Meier) of the long-term outcome of surgical AS treatment and of the predictive value of clinical parameters of knee joint involvement was also performed. No intra- or post-operative morbidity, pain worsening or loss of joint motion was observed and all patients were discharged within 48 h. Comparison of the parameters of knee joint evaluation showed a significant reduction of the signs of joint inflammation and a significant increase in the ROM in all follow-up periods. At 36 months, the survival curves showed a 61.2% cumulative probability of clinical remission and 72.8% of definite improvement. No significant differences in the prognostic importance of RA, compared to PsA diagnosis, were observed, although higher percentages of PsA compared to RA knees (86.3% and 45.7% respectively) reached the end point of clinical remission at 36 months. KJS duration, radiographic severity and cartilage damage were not predictors of poor long-term outcome of AS synovectomy. Surgical AS treatment for PsA knees with more advanced cartilage damage gave a better long-term outcome. A total of 50.7% of operated knees reached the end point of a KJS relapse at 36 months, the majority (82%) within the initial 18 months of follow-up. Our study indicates that AS synovectomy is a safe procedure requiring short hospitalization which, in combination with second-line medical treatment, can reduce local inflammation in RA and PsA KJS, and preserve knee joint ROM for up to 3 yr.