From 1990 through to 1997, 105 pregnant women with typical EM were investigated at the Lyme Borreliosis Outpatients' Clinic of the Department of Infectious Diseases at the University Medical Centre in Ljubljana, Slovenia. Twenty-five (23.8%) patients acquired borrelial infection during the first trimester of pregnancy; eight (7.6%) of them had noticed the skin lesion before they became pregnant. In 43 (40.6%) patients the EM appeared in the second trimester, and in 37 (35%) patients in the third trimester of pregnancy. Two (1.9%) patients received phenoxymethyl penicillin (1 million IU t.i.d.), three (2.9%) benzyl penicillin (10 million IU b.i.d.), and 100 (95.2%) ceftriaxone (2 g daily). All patients were treated for 14 days except three (2.9%) in whom the treatment with ceftriaxone was discontinued because of mild side effects. The outcome of disease was good in all 105 patients. Ninety-three (88.6%) out of 105 patients had normal pregnancies; the infants were delivered at term, were clinically healthy, and subsequently had a normal psychomotor development. In the remaining 12 (11.4%) patients an adverse outcome was observed. Two (1.9%) pregnancies ended with an abortion (one missed abortion at 9 weeks, one spontaneous abortion at 10 weeks), and six (5.7%) with preterm birth. One of the preterm babies had cardiac abnormalities and two died shortly after birth. Four (3.8%) babies born at term were found to have congenital anomalies; one had syndactyly at birth and three had urologic abnormalities which were registered at the age of 5, 7, and 10 months, respectively. A causal association with borrelial infection was not proven in any infant. For at least some unfavourable outcomes a plausible explanation not associated with Lyme borreliosis was found.