Marfan syndrome (MFS) is an inherited connective tissue disorder mainly caused by the fibrillin-1 mutation. Deficient fibrillin-1 is thought to result in the failed sequestration of transforming growth factor β (TGFβ) and subsequent activation of the TGFβ signaling pathway, suggesting that the circulating TGFβ level may be elevated in MFS, although its accurate measurement is complex due to ex vivo release from platelet stores upon platelet activation. We measured the plasma TGFβ1 levels of 32 Japanese MFS patients (22 medically untreated, 10 treated, 20 males, 30.1 ± 9.6 years old) and 30 healthy volunteers (19 males, 29.5 ± 5.8 years old) by ruthenium-based electrochemiluminescence platform (ECL). PF4 was also measured by enzyme immunoassay (EIA) as a platelet degranulation marker. There was no significant difference in the mean plasma TGFβ1 level between the MFS group (1.31 ± 0.40 ng/mL) and controls (1.17 ± 0.33 ng/mL) (P = 0.16, NS). Also, there was no significant difference between the untreated (1.24 ± 0.37 ng/mL) and treated (1.46 ± 0.45 ng/mL) MFS patients (P = 0.15, NS). We also measured PF4, which showed wide deviations but no significant difference between the two groups (P = 0.50). A difference in circulating TGFβ1 levels between MFS patients and controls was not detected in this Japanese population. Circulating TGFβ1 is not a diagnostic and therapeutic marker for Japanese MFS patients, although our findings do not eliminate the possible association of TGFβ with the pathogenesis of MFS.