DISCUSSION: In summary, this case presents rapid progression of PAH within the timespan of a year. Common causes of PAH such as obstructive lung disease or chronic left heart failure typically result in a much slower disease course and rarely cause such elevated PAP. Hence, the best explanation for the rapidity and severity of PAH in this case can be attributed to a drug-effect from rituximab, which he had received in the preceding months. Another learning point is that vasodilator testing during a RHC can provide a potential treatment option, which is valuable for a disease such as PAH where options tend to be limited. Although cardiac output declines from 5.9 to 5.75 L/min, this was not felt to be significant and the vasodilator test was still considered to be positive.