PKU (phenylketonuria) is a genetic disorder that causes severe cognitive impairment if it is not detected and treated with a strict and difficult diet. In a lifetime of practice, most physicians will never encounter a single case of PKU, yet every physician in the industrialized world learns about the disease in medical school and, since the early 1960s, the newborn heel stick test for PKU has been mandatory in many countries. Diane B. Paul and Jeffrey P. Brosco’s beautifully written book explains this paradox. The development of state programs for early detection of and treatment for PKU is deservedly considered a great public health success story. Advocates have traded on this success to urge expanded newborn screening, to defend basic research in genetics, and to confront proponents of genetic determinism. When deployed for these purposes, treatment for PKU is typically represented as a simple matter of adhering to a low-phenylalanine diet. In reality, the challenges of living with PKU are daunting. In this first general history of PKU, a historian and a pediatrician explore how a rare genetic disease became the object of an unprecedented system for routine testing. The PKU Paradox is informed by interviews with scientists, clinicians, policy makers, and individuals who live with the disease. The questions it raises touch on ongoing controversies about newborn screening and what happens to blood samples collected at birth.