A novel type platelet aggregometer, a WBA Analyzer, has enabled us to obtain the platelet aggregability data immediately after blood sampling, which is considered to closely reflect in vivo platelet function. Using this analyzer, we measured the platelet aggregatory threshold index (PATI) 5 min after blood sampling and compared it with that 60 min after blood sampling in 20 healthy male volunteers (10 smokers and 10 non-smokers). In the non-smokers, PATI was 10.3 ±2.3 μM 5 min after blood sampling, and it decreased to 4.7 ±1.5 ( P <0.001) 60 min after blood sampling. In the smokers, the PATI was 7.7 ±2.9 μM 5 min after blood sampling, and it decreased to 3.8 ±1.5 ( P <0.001) at 60 min after blood sampling. In the smokers, the PATI 5 min after blood sampling increased after a 4-week cessation of smoking (10.4 ±2.9, P <0.01), although the PATI 60 min after blood sampling did not change (4.2 ±1.6 μM). The measurement of platelet aggregability immediately after blood sampling using a WBA Analyser may be useful to evaluate not only platelet function in various thrombotic disorders, but also the effects of various anti-platelet drugs. Cessation of smoking should also be encouraged in the light of the adverse effects on platelet function.