Histopathological examination of synovial specimens can contribute to the diagnosis of chronic joint diseases. A so-called synovitis score has been introduced as a standardised grading system, based on the semi-quantitative evaluation of the three determining features of chronic synovitis: enlargement of synovial lining, density of synovial stroma and inflammatory infiltrate, giving a score between 0 and 9. The present study examines the reliability of this procedure by comparison with exact measurements using computer-assisted image analysis (CAIA). Seventy-one synovial specimens from patients with osteoarthritis (OA, n = 22), psoriatic arthritis (PsA, n = 7), rheumatoid arthritis (RA, n = 35) and from a control group (Co, n = 7) were evaluated using both the synovitis score and CAIA. The measurements were transformed to semi-quantitative values analogous to the synovitis score. The differences between the transformed CAIA scores and the pathologist’s scores were 0 or +/−1 in 40 cases, whereas in 31 cases the difference was greater than 1 (correlation coefficient r = 0.725). The CAIA scores differed significantly between Co and RA cases (p = 0.000) as well as between OA and RA (p = 0.000). We conclude that the synovitis score was validated by CAIA and can be regarded a reliable grading system that contributes to the diagnostic procedure of chronic joint inflammation.