This technical paper examines the issue of attrition bias in two cohorts of the Longitudinal Surveys of Australian Youth (LSAY), based on an analysis using data from 1995 to 2002. Data up to 2002 provided eight years of information on members of the Y95 cohort and five years of information on members of the Y98 cohort. This amount of time was considered adequate to evaluate the extent of attrition bias and the performance of weights in correcting for bias. LSAY was designed to explore the transitions made by these cohorts of young people as they leave school and enter the labour force, engage in further study and become adults. It focuses on outcomes and how earlier factors may have influenced those outcomes. At the time each cohort was drawn, the sample represented the population of 15 year-old Australian students attending Australian schools, but like other longitudinal studies, LSAY experiences attrition of its respondents. The weighting schema was designed to ensure that remaining members of the cohort represented the original cohort, not to represent the population of young people in subsequent years. The goals of this technical paper are: understanding the extent of attrition in the 1995 and 1998 Year 9 LSAY cohorts, calculating the amount of bias caused by attrition in these cohorts and determining whether the current practice of calculating weights is appropriate or additional practices are required to ameliorate problems caused by attrition in LSAY.