The authors suggest that the two conditions may share common genetic or environmental causes.
The study involved 2.27 million people who were born in Denmark between 1950 and 1987, and were identified from national registers. Personal and family histories of epilepsy and psychosis were obtained, and individuals were monitored for up to 25 years.
The team found that people with a history of epilepsy had nearly two and a half times the risk of developing schizophrenia and nearly three times the risk of developing a schizophrenia-like psychosis compared with the general population. The risk was the same for men and women but increased with age.
Both a family history of epilepsy and a family history of psychosis were also significant risk factors for schizophrenia and schizophrenia-like psychosis. For epilepsy, however, the increased risk was more pronounced for people with no family history of psychosis.
The increased risk did not differ by type of epilepsy, but was significantly greater the older people were when they were first admitted to hospital for epilepsy.
"We think that this study is the first, on a population level, to show that a family history of epilepsy increases the risk of schizophrenia or schizophrenia-like psychosis even after adjusting for the effects of personal history of epilepsy and other factors," say the authors.
"This finding suggests that genetic or environmental factors shared by family members may have an important role."