UNLABELLED Excretion of radiopharmaceuticals into breast milk poses a potential risk to infants and clear recommendations regarding interruption times are required. There are few data available regarding the impact of (18)F-FDG on this issue. With increasing use of PET for oncologic imaging and its potential advantages to nursing mothers because of its short physical half-life compared with other commonly used tumor imaging agents such as (67)Ga and (201)Tl, evaluation of the excretion pattern of this agent in breast milk is important. METHODS We have evaluated the uptake of FDG in the breasts in 7 women, 6 of whom were lactating and 1 of whom was in early postpartum but had not commenced breast-feeding. Milk samples were obtained from 4 of the lactating women, including serial samples from 1. RESULTS Significantly increased breast uptake was identified in all lactating breasts but not in 1 breast consistently refused by the nursing infant or in the woman who had not begun breast-feeding after delivery of her child. No qualitative change or semiquantitative estimate of radiotracer uptake in the breast was seen after expression of breast milk. Decay-corrected activity measurable in breast milk ranged from 5.54 to 19.3 Bq/mL/MBq injected. Using a standard model of breast-feeding, the calculated maximum cumulative dose to the infant, 0.085 mSv with no interruption of breast-feeding, is well below the recommended limit of 1 mSv. CONCLUSION High uptake of FDG in the lactating breast appears to be related to suckling. There is, however, little secretion of activity into breast milk. Accordingly, a higher radiation dose is received by the infant from close contact with the breast than from ingestion of radioactive milk.