In the present study, attitudes toward organ donation in transplant patients and their relatives, non-transplant patients (cancer, heart disease etc.) and healthy people were analyzed on the basis of the Theory of Planned Behavior (TPB - Ajzen, 1985). Data was collected by the personal information form and the attitudes toward organ donation questionnaire based on the TPB. The questionnaire was developed by using the conceptual and methodological considerations as recommended by Ajzen (2002). Transplant patients and their relatives got higher scores compared to other two groups in the TPB's antecedents of attitude, subjective norm, perceived behavioral control and intention. At the end of the study, participants were asked if they would agree to sign an organ donation form. Participants who accepted to fill out an organ donation form were found to have more positive attitudes toward organ donation, perceived more control over behavior, and felt more social pressure compared to the ones who did not agree to fill out the form. TPB assumes that the attitude, subjective norm, perceived behavioral control and intention are the antecedents of behavior. The contribution of these antecedents on the prediction of organ donation behavior was analyzed by structural equation modeling. In the structural analysis of the data collected on the basis of TPB, it was observed that perceived behavioral control mediated the relationships between attitude-intention, and subjective norm-intention.