Preliminary Japanese diagnostic criteria for the classification of mixed connective tissue disease (MCTD) were tested in a group of 32 Caucasian patients with this disease. Many clinical and laboratory similarities were found between Caucasian and Japanese patients. However, polyarthritis was more frequent in the Caucasians, while finger and hand swelling, DLCO reduction and muscle involvement were more frequent in the Japanese. In Caucasians the sensitivity of this criteria set was 87%, very similar to that found in the Japanese group (88%), and the specificity was 94%, higher than that of Japanese (87%). The difference resulted from the higher specificity of anti-nRNP antibody positivity in the Caucasian patients, probably due to the use of counterimmunoelectrophoresis in the detection of this antibody. The Japanese criteria seem more useful than others because they allow the use of techniques other than passive hemagglutination in detecting the anti-nRNP antibody. In our experience, such criteria also contribute to a better definition of MCTD in Caucasian patients.