In this study, we determined the relationship between Guillain-Barré syndrome (GBS) and pregnancy. By taking advantage of several nationwide registers and the availability of personal identification numbers, we calculated person-years for Swedish females aged 15-49 years in the following categories: (1) neither pregnant nor postpartum; (2) pregnant; (3) in the first month postpartum, or (4) in the first 3 months postpartum during 1973-1983. For these women, we determined the corresponding exposure status of hospital-registered GBS cases. Medical records were examined for GBS cases hospitalized during the 2-week period postpartum and 1-month period after the last menstruation. Poisson regression analysis yielded age-adjusted relative risks (RRs) of 0.86 (95% CI 0.40-1.84) for pregnant women, and 1.47 (0.54-3.99) and 2.21 (0.55-8.94) for females during the 3-month and the 30-day period after delivery. The risk for GBS seems to be lower during pregnancy and increases after delivery.