Inhalation of Stachybotrys chartarum, a ubiquitous fungus in our living environment, has been suspected as a cause of acute idiopathic pulmonary haemorrhage in infants, but its relation to human diseases is not yet known. The aim of present study was to investigate the effect of repeated intratracheal injection of the fungus into mice, paying special attention to the pulmonary vascular system. Spores of S. chartarum were injected into the trachea of mice from 6 to 18 times over 4-12 weeks, and the lungs were examined by histopathology, morphometrics and haemodynamics. When 1 x 10(4) spores/mouse were injected, histopathological examination showed the development of pulmonary arterial hypertension (PAH). Symmetrical thickening of the intima and media of the pulmonary arterial walls was seen after six injections over 4 weeks. Right ventricular hypertrophy was also evident after 12 injections. PAH was confirmed by the elevation of right ventricular systolic pressure (20.1 +/- 5.7 mmHg in the injected group vs. 12.0 +/- 2.4 mmHg in the control group, P < 0.01). This study showed that the inhalation of S. chartarum caused PAH in mice, suggesting a potential of S. chartarum as a cause of human health problem such as PAH.