Objectives: To estimate 1) the risk of candidiasis (neonatal thrush or breast infections or both) in nursing mother-infant pairs and 2) whether receipt of intrapartum antibiotics increases this risk. Methods: Demographic and obstetric data were obtained at delivery, and telephone follow-up was obtained at 1 week and 1 and 3 months and recorded in the Lactation Services database, which was reviewed retrospectively. The diagnoses of thrush and breast candidiasis were based on symptoms and patient reports, as per standard clinical practice. For statistical analysis, χ 2 analysis and Student t test were used. A value of P < .05 was considered significant. Results: A total of 811 nursing mother-infant pairs were seen between February 1, 2001, and August 31, 2002. Mother-infant pairs with follow-up who nursed for 1 month or longer were included (n = 435). Of these, 173 (39.8% ) received intrapartum antibiotics, most (78.6% ) for group B streptococci prophylaxis. Overall, 46 mother-infant pairs (10.6% ) had either thrush or breast candidiasis (32 with both) within 1 month of delivery. Mothers who were exposed to intrapartum antibiotics were significantly more likely to develop breast candidiasis (odds ratio 2.1, 95% confidence interval 1.08-4.08). Antibiotic-exposed neonates were more likely to develop thrush, although this was not statistically significant, (odds ratio 1.87, 95% confidence interval 0.97-3.63). Antibiotic-exposed infants were younger (mean ± .standard deviation, 38.5 ± .1.9 weeks compared with 39.0 ± .1.3 weeks; P = .002), but there were no differences in maternal age, gravity or parity, or route of delivery. Conclusions: Neonatal thrush and maternal breast candidiasis are common early postnatal complications. The higher rates of thrush and breast candidiasis in antibiotic-exposed mother-infant pairs merits further study. Objectives: To estimate 1) the risk of candidiasis (neonatal thrush or breast infections or both) in nursing mother-infant pairs and 2) whether receipt of intrapartum antibiotics increase this risk. Methods: Demographic and obstetric data were obtained at delivery, and telephone follow-up was obtained at 1 week and 1 and 3 months and recorded in the Lactation Services database, which was reviewed retrospectively. The diagnoses of thrush and breast candidiasis were based on symptoms and patient reports, as per standard clinical practice. For statistical analysis , χ 2 analysis and Student t test were used. A value of P <.05 was considered significant. Results: A total of 811 nursing mother-infant pairs were seen between February 1, 2001, and August 31, 2002. Mother-infant pairs with follow-up who nursed for 1 month or longer were included (n = 435). Of these, 173 (39.8%) received intrapartum antibiotics, most (78.6%) for group B streptococci prophylaxis. Overall, 46 mother-infant pa Mothers who were exposed to intrapartum antibiotics were significantly more likely to develop breast candidiasis (odds ratio 2.1, 95% confidence interval 1.08-4.08) irs (10.6%) had either thrush or breast candidiasis (32 with both) within 1 month of delivery Although antibiotic-exposed neonates were more likely to develop thrush, although this was not statistically significant, (odds ratio 1.87, 95% confidence interval 0.97-3.63). Antibiotic-exposed infants were younger (mean ± .standard deviation, 38.5 ± .1.9 weeks compared with 39.0 ± .1.3 weeks; P = .002), but there were no differences in maternal age, gravity or parity, or route of delivery. Conclusions: Neonatal thrush and maternal breast candidiasis are common early postnatal complications. of thrush and breast candidiasis in antibiotic-exposed mother-infant pairs merits further study.