Ivermectin is a widely used drug for the treatment of various neglected tropical diseases, such as lymphatic filariasis, onchocerciasis, and strongyloidiasis among others. Despite its excellent safety profile, there are few published studies of the use of ivermectin in children, pregnant and nursing women. In the present study, we report clinical data on ivermectin concentrations in breastmilk of a woman with Strongyloides stercoralis and HTLV-I coinfection. Ivermectin levels in breastmilk ranged from 1.4 to 20.8 ng/ml, with a mean of 9.26 ng/ml after a single dose of 200 µg/kg. We estimated the possible ivermectin exposure of the infant to be 1.1 µg/kg, 0.55% of the weight-adjusted percentage of the maternal dose. This value is largely under the threshold established by the World Health Organization for safe breastfeeding. Our results bolster previous findings on the secretion of ivermectin into breastmilk in healthy volunteers. The findings from this case study do not support exclusion of lactating women or interrupting lactation to accommodate it.