We examined the extent to which program integrity (i.e., the degree to which programs were implemented as planned) was verified and promoted in evaluations of primary and early secondary prevention programs published between 1980 and 1994. Only 39 of 162 outcome studies featured specified procedures for the documentation of fidelity. Of these, only 13 considered variations in integrity in analyzing the effects of the program. Lowered adherence to protocol was often associated with poorer outcome. There was mixed evidence of dosage effects. The omission of integrity data, particularly measures of adherence, may compromise the internal validity of outcome studies in the prevention literature. We do not view procedures for integrity verification as inconsistent with the adaptation of interventions to the needs of receiving communities.