To the Editor: In his letter, Dr. Napoli raises a series of very important questions regarding the putative etiology of vulnerability factors for PTSD. Each of the questions raised is empirical and can be addressed with systematic research. Ongoing work in our laboratory aims to elucidate differences between vulnerable and less vulnerable children of Holocaust survivors by exploring psychological and biological characteristics of the offspring as well as their parents. Dr. Napoli is correct in underscoring the importance of adoption studies, which, although difficult to perform, would shed light on the nature as opposed to nurture aspects of the intergenerational transmission of PTSD.