Breast-feeding is considered the gold standard for infant nutrition. In spite of statements about the safe use of drugs in lactation by the American Academy of Pediatrics, medical professionals remain confused regarding the management of drug therapy in nursing mothers, and this can lead to suboptimal prescribing and poor compliance. The aim of our study was to evaluate the safety of 2 of the newer antibiotics, amoxicillin/clavulanic acid and cefuroxime, during lactation. Breast-feeding women who called a drug consultation center to obtain information about the potential risks of amoxicillin/clavulanic acid (67 women) and cefuroxime (38 women) were prospectively recruited. As a control group, women who were treated with antibiotics known to be safe during lactation were recruited: amoxicillin (n = 40) for the amoxicillin/clavulanic acid group and cephalexin (n = 11) for the cefuroxime group. Women in the control group were matched for indication for antibiotic therapy, duration of treatment, and maternal age. Participants were interviewed after treatment termination regarding adverse reactions during therapy. In the amoxicillin/clavulanic acid group, 15 infants (22.3%) had adverse effects, and the rate increased with dosage (P = 0.0139). This was significantly higher than the amoxicillin group, where 3 infants (7.5%) had adverse effects (P = 0.046, relative risk (RR) = 2.99, 95% confidence interval (CI) 0.92-9.68). However, there were no significant differences between rates of specific events. The rate of adverse effects in the cefuroxime group (2.6%) was not significantly different from that in controls (9%) (P = 0.58, OR = 0.92, 95% CI 0.94-1.06). All adverse effects were minor, self-limiting, and did not necessitate interruption of breast-feeding. Our data suggest that amoxicillin/clavulanic acid and cefuroxime may be safe during lactation. Larger studies are needed to confirm these findings.