Little is known about infant’s safety of chemotherapy during breastfeeding where evidence is limited to a few case reports. This lack of knowledge has led to a general tendency to advise against breastfeeding during cytotoxic therapy despite the overwhelming benefits that breastfeeding offers to both the mothers and their children. In this case series, the presence of five chemotherapies in breast milk was determined. The aim was to obtain insight into the presence of these drugs in breast milk to inform and help clinicians in making informed decisions for women who want to breastfeed. Three patients collected 24-hour samples of breast milk every day for 1, 2, or 3 weeks after chemotherapy, 210 in total. After determination of drug concentrations, the infant daily dose, relative daily infant dose (RID%) and cumulative RID were calculated. Cumulative RIDs in patients varied from 10% to values lower than 1%. Rich data allowed us to design a table which gives predictions on the amount of days that breast milk has to be discarded to reach cumulative RIDs below 5, 1, and 0.1% for each compound. For cyclophosphamide, paclitaxel, and carboplatin, cumulative RIDs below 1 or 0.1% are reached if breast milk is discarded for 1–3 days after administration. This might suggest that breastfeeding in between cycles is an option. However, other pharmacological parameters should also be taken into consideration. For doxorubicin, also the levels of the active metabolite doxorubicinol need quantification. Similarly, breastfeeding during treatment with cisplatin might give substantial exposure and we advise caution.