[2] After discovering a keen interest in mathematics, he began working in epidemiology, specializing in air pollution, initially as a summer student at the California State Health Department.

[3] It was here that he first conceived the idea of doing a cohort study of women's health, initially looking into the effects of oral contraceptives on a population of doctors' wives.

[4] In 1976, inspired by his work in England, Speizer decided to conduct a large study of women's illnesses, notably cancer and heart disease, using a cohort of 120,000 nurses whose health would be monitored over the following years and decades.

This became the first Nurses' Health Study, which spawned "hundreds of scientific papers... covering scores of diseases".

[5] In a 2011 interview with Douglas Dockery, Speizer listed his research interests as "the natural history of chronic respiratory disease and... the components that cause the disease to get worse" and "the natural history of what happens as people get older, and the interaction with lifestyle and behavioural factors".