An internal validation substudy compares an imperfect measurement of a variable with a gold standard measurement in a subset of the study population. Existing guidance on optimal sampling for validation substudies has assumed complete enrollment and follow-up of a target subset with fixed size. The adaptive validation approach provides a framework to monitor validation data as they accrue until predefined stopping criteria for the goals of validation are met. The method provides an approach to effective and efficient estimation of classification parameters as validation data accrue and can be modified to meet alternative criteria given specific study or validation study objectives.