We report 4 cases of pseudothrombocytopenia due to platelet cold agglutinins. Case 1 was a 57 y.o. female whose platelet count was 97 x 10(3)/microl. Case 2 was a 37 y.o. male with a platelet count of 96 x 10(3)/microl. Case 3 was a 74 y.o. male with a platelet count of 28 x 10(3)/microl. Case 4 was a 62 y.o. female whose platelet count was 34 x 10(3)/microl. The platelet counts in these 4 cases were decreased and blood smears showed platelet clumping in blood drawn in a tube without anticoagulant just after withdrawal, as well as in blood drawn in a tube with anticoagulant. The platelets from these patients agglutinated at a temperature below 10 degrees C (case 1 and 4) and 24 degrees C (case 2). The immunoglobulin class of the platelet cold agglutinins in cases 1, 2 and 4 was IgM. Agglutinated platelets showed no activation marker, such as CD62P, CD63 or CD40L, on the surface of the platelets. The target antigen of cold agglutinins was GPIIb-IIIa in cases 1 and 2. We considered that the detection of platelet agglutination in blood without anticoagulant is important to diagnose pseudothrombocytopenia due to platelet cold agglutinins. Although this disease is considered to be very rare, we suspect that this disease may be misdiagnosed as pseudothrombocytopenia due to the presence of an anticoagulant, and overlooked.