We report results from a study of pre and post assessments of students enrolled in reformed and non-reformed introductory physics laboratories. This study assesses the impact of Florida International University's (FIU) PhysTEC (Physics Teacher Education Coalition) reform of introductory physics labs. Prospective pre-service teachers were trained and placed in six lab sections serving as undergraduate Learning Assistants (LAs) and implementing tutorial-based curriculum. LAs facilitated epistemological discussions designed to challenge and then refine student understanding of physics concepts. Students completed the Force Concept Inventory (FCI) , the Maryland Physics Expectation Survey (MPEX 2)  , and common exam questions embedded in the exams for their physics classes. We find significant differences in normalized gain on the FCI and common exam questions in favor of students in the reformed labs. There was no significant difference in pre and post MPEX 2 scores for reformed lab students, generally agreed to be a positive outcome. © 2008 American Institute of Physics.