Fourteen case-control studies evaluating the importance of different risk factors for infection of humans with Toxoplasma gondii were identified. Surprisingly, up to two-thirds of infections could not be explained by risk factors such as consuming raw or undercooked meat or poor kitchen hygiene. Few studies reported population attributable risk, and when reported, it was low. Protocols including questionnaires used for collecting data, age and socio-economic groups studied, and sample sizes varied among studies. There is a need for carefully planned, case-control studies to identify sources of infection with T. gondii in humans. New risk factors, such as the manufacture of organic meat, and possible ways to sterilize meat to inactivate T. gondii tissue cysts are discussed.