Objectives Minimizing attrition is a key objective in longitudinal research, with possible consequences being additional bias and reduced generalizability. Identifying determinants of attrition is essential in determining attrition prevention strategies. The objective of this study was to investigate a number of these determinants, with an emphasis on contactability. Study Design and Setting Data were taken from the Passports project, a randomized controlled trial of an intervention to provide postrelease support to ex-prisoners in Queensland, Australia. Measures of contactability included intervention intensity, baseline collaterals, and follow-up telephone calls, with attrition at follow-up being the outcome event. Multivariable modeling was used to assess the independent effects of these measures on attrition. Results Attrition was found to be more likely among those who were not contacted between the release and follow-up (adjusted odds ratio [AOR]: 2.93; 95% confidence interval [CI]: 1.87, 4.60), did not provide collaterals (AOR: 2.58; 95% CI: 1.68, 3.97), and received more than four telephone calls (AOR: 2.42; 95% CI: 1.61, 3.63). Evidence of dose-response relationships between attrition and the measures of collaterals and telephone calls was also seen to exist. Conclusion These findings have implications for sample size maintenance, especially those involving hard-to-reach populations. Subject to cost constraints and possible diminishing returns, researchers should endeavor to implement a study protocol that facilitates continued contact during follow-up.