A growing body of scientific literature implicates toxic exposures in childhood illnesses and developmental disorders. When these illnesses and disabilities result from environmental factors under human control, they can and should be prevented. This report documents monetary costs associated with five major areas of health problems in children that have been linked to preventable environmental exposures: cancer, asthma, lead poisoning, neurobehavioral disorders, and birth defects. We review incidence and prevalence estimates for these disorders, as well as estimates of the associated monetary costs. We apply the concept of the "environmentally attributable fraction" (EAF) of an illness, where EAF is the estimated percentage of cases of an illness that result from an environmental exposure. Preventable childhood illnesses and disabilities attributable to environmental factors are associated with large monetary costs. Our estimate of direct and indirect costs ranges from $1.1 to $1.6 billion annually in Massachusetts. Of course, there is no dollar measure of the full practical and emotional burden borne by these children, their families, and the communities in which they live.