Background: The autoimmune process later leading to type 1 diabetes (T1D) seems to start very early in life. Different viruses have been suspected to contribute to the development of T1D, some already during pregnancy. As viruses may be hosted by animals and from them transferred to humans we decided to investigate if exposure to pets during pregnancy is related to later development of T1D. Methods: ABIS (All Babies in Southeast Sweden)-is a prospective population-based cohort study of unselected children born in southeast Sweden between Oct first 1997 to Oct first 1999. Parents of 16384 children answered a questionnaire within 3 days after birth including information about exposure to different pets. The ABIS registry has been connected to the National Registry of diagnosis and also the national Registry of Drug prescriptions so we know that 137 children have got T1D, and they were compared with the non-diabetic population. Results: During pregnancy, 45.5% of the mothers had pet animals at home. Most common were cats (25.0%) and dogs (18.7%). Neither exposure to dogs (OR = 1.27, P = 0.23) or cats (OR = 0.81, P = 0.31) were associated to later T1D risks. However, exposure to hamsters increased the T1D risk (OR 4.21, P = 0.0007). In a multiple regression this association remained (P = 0.005) when adjusted for other possible risk factors. Conclusions: Exposure to hamster during pregnancy seems to increase the risk of T1D in the child. One possibility could be infection by virus hosted by the pet.