Advanced paternal age has been widely cited as a risk factor for schizophrenia among offspring and even claimed to account for one-quarter of all cases. We carried out a new study on 25,025 offspring from the Collaborative Perinatal Project (CPP), including 168 diagnosed with psychosis and 88 with narrowly defined schizophrenia. We also conducted a meta-analysis of this and nine other studies for which comparable age-cohort data were available. The mean paternal age for the CPP cases was slightly, but not significantly, higher than the matched controls (p=0.28). Meta-analyses including these new results were conducted to determine the relative risk associated with alternative definitions of advanced paternal age (35, 45 or 55 years and older). These yielded pooled odds ratios and 95% confidence intervals of 1.28 (1.10, 1.48), 1.38 (0.95, 2.01) and 2.22 (1.46, 3.37), respectively. Thus, increased paternal age appears to be a risk factor for schizophrenia primarily among offspring of fathers ages 55 and over. In these 10 studies, such fathers accounted for only 0.6% of all births. Compared with other known risk factors for schizophrenia, advanced paternal age appears to be intermediate in magnitude. Advanced paternal age is also known to be a risk factor for some chromosomal and neoplastic diseases in the offspring where the cause is thought to be chromosomal aberrations and mutations of the aging germline. Similar mechanisms may account for the relationship between advanced paternal age and schizophrenia risk.