The paper examines the validity of two indices of sustainability: family planning programme sustainability (PSI) and outcome sustainability (OSI) developed by Tsui and Knight (1997) by applying their original method to recent data. The indices succeed in identifying the directional path of programme and outcome sustainability. Close correlations are found between PSI and OSI predicted values and actual programme and outcome values. The indices provide a repeatable method for measuring sustainability, although they are sensitive to data measurement errors. The indices provide a policy tool for funding decisions but should be used with other data sources to judge sustainability.