The Problem-Based Learning (PBL) Project for Teachers included three days of professional development using PBL to deepen teachers' content knowledge. Teachers identified the content area in which they wished to work from a list of choices. They worked in facilitated groups of from five to nine participants to solve rich, ill-structured problems. Content knowledge was assessed based on teachers' written responses to at least one general question and one application question in which they explained some everyday phenomena. Teachers responded to these questions two months before and at the end of the professional development; 80.5 percent of the 41 middle and high school teachers who participated showed improved content knowledge. We also examined the patterns in teachers' incoming content knowledge and changes in content knowledge. The data indicate that the PBL approach used here was effective in improving the content knowledge of teachers independent of demographic factors and type and degree of incoming content knowledge.