Attentional deficits are well-established characteristics of patients with schizophrenia and their at-risk offspring, suggesting a biological connection between attention and schizophrenia. The goal of this study is to clarify the developmental role of attention in the illness. Data has been collected from 87 subjects at high and low risk for schizophrenia who have participated in the New York High-Risk Project from 1977 to the present. Individuals are considered to be at high risk if either or both of their parents has schizophrenia. Analyses of attention and global behaviors, measured at intervals from about 12 to 26 years of age, indicate (a) attentional deficits can be reliably detected in high-risk children who will develop future schizophrenia-spectrum disorders (the prespectrum [PSP] group); (b) these deficits are stable, enduring over time, and appear to reflect a compromised attentional capacity; (c) attention is not affected by the onset of illness in the PSP group; (d) for all subjects, attention and global behaviors follow independent developmental pathways; and (e) behavioral difficulties, but not attention deficits, appear to be highly sensitive to environmental factors, especially rearing by a mentally ill parent. It is concluded that in PSP individuals impaired attention probably results from prenatal developmental abnormalities (possibly on the cellular level) and is likely to be a marker of a biological vulnerability to schizophrenia. In addition, attentional deficits, as opposed to early behavioral difficulties, are concluded to be a useful first step in screening for youngsters in need of early intervention.