To the Editor: We read with great interest the article by Carole Di Maggio, M.D., and her colleagues (1). Although the study presented some interesting data about decreasing age at onset of schizophrenia, it also raised some questions about the interpretation and analyses of the data. The authors presented retrospective data for medical records (N=877) from 1976, 1986, and 1996. Individuals in the study were divided into three birth cohorts: 1905–1944, 1945–1964, and 1965–1984. However, the significant decrease in age at onset through time could be due to bias caused by the different age structures of the cohorts. Individual ages of the subjects in the 1905–1944 cohort were from 32 to 91 years, the subjects in the 1945–1964 cohort were aged 12 to 51 years, and the subjects in the 1965–1984 cohort were aged 1 to 31 years. Thus, all persons who died before the age of 32 could not be part of the 1905–1944 cohort. This might have been an important source of bias in the study because mortality is greater for patients with schizophrenia, but at the population level, the standardized mortality ratio for schizophrenia decreases exponentially with age (2). Only patients with illness onset before age 32 could be present in the youngest cohort.