Plasma thromboxane B₂ (TXB₂), cyclic AMP and cyclic GMP Ievels in 31 women with Takayasu disease were measured and findings were compared with data on 34 age-matched healthy Japanese. Platelet aggregation determined by the screen filtration pressure (SFP) method and pulmonary lesions assessed by scintillation were studied in 21 patients. Plasma TXB₂ levels in patients with Takayasu disease were 265.8 ± 20.4 pg/ml, which were statistically higher than the levels of 196.3 ± 10.7 in healthy controls. On the contrary, cyclic AMP Ievels in the patients were 9.6 ± 0.7 pmols/ml, statistically lower levels than 13.5 ± 1.0 seen in the healthy controls. A positive correlation among cyclic AMP, cyclic GMP and TXB₂ Ievels was confirmed only in the controls. SFP values in the patients with pulmonary lesions were significantly higher than in those without lesions and also in the controls. A positive correlation was obtained between SFP and TXB₂ Ievels in healthy controls, but not in the patients. Thus, thromboxane A₂ may play an important role in the pathology of Takayasu disease.