The present invention provides novel methods of inhibiting fibrosis, as well as methods of treating or inhibiting fibrotic disorders, using BMP9 and/or BMP10 antagonists. The present invention also provides methods of assessing whether a subject has or is at risk of developing a fibrotic disorder by detecting levels of BMP9 and/or BMP10. Further provided are methods of assessing the efficacy of a treatment regimen for treating a fibrotic disorder by detecting and comparing pre-treatment levels of BMP9 and BMP10 with post-treatment levels of BMP9 and BMP10.