This paper presents findings of a case study that documented the implementation of a portfolio assessment system in response to mandated program improvement and assessed its impact on teacher and student behaviors. The sample included elementary and middle school teachers and students from three Chapter 1 schools in a rural California school district. The schools had been identified for mandatory program improvement and decided to implement a portfolio assessment system to increase student learning. Data were collected through a teacher questionnaire, teacher interviews, monthly onsite visits, and document analysis. A conclusion is that if Chapter 1 programs use portfolios as an alternative form of assessment, teacher concerns become important. Organizational, logistic, and teacher-behavior factors that affect portfolio implementation are identified. Organizational factors include district support, staff development, the role of portfolios in student assessment, and parental support. Logistical factors include time, portfolio management, and rubric development. Finally, portfolios may impact teachers' instructional practices. Criteria for validating portfolio assessment include cost, efficiency, and generalizability. (LMI)