This paper reports on a one year, mixed-methods longitudinal case study investigating the neglected area of the perceived reasons why participants forget vocabulary knowledge. The participants were 43 fourth year male Saudi EFL majors at King Abdulaziz University KAU, Saudi Arabia. Quantitative and qualitative data including self-reported questionnaires and retrospective semi-structured interviews offered evidence to support the findings of this study. The reasons associated with lexical attrition centered on lack of practice, instructional and environmental context and nature of the word.