To investigate the potential effects of maternal infection during lactation on breast-milk quantity and composition, we examined low-income Peruvian women who had an acute febrile infection and were exclusively breast-feeding a child from 1 to 6 mo of age (n = 36). Women who were not ill (n = 38) served as controls; all women had body mass indexes (in kg/m2) > 19.5. Blood and milk samples were collected on days 1, 7, and 14 after identification of the episode of illness. C-reactive protein in maternal serum was significantly elevated by infection, whereas two other acute-phase reactants, ceruloplasmin and α2-macroglobulin, showed no change. Serum zinc concentrations were significantly lower in ill women than in women who were not ill, whereas serum copper concentrations were elevated initially in ill women. Serum iron concentrations increased significantly with time, but there was no significant difference between groups. Milk intake, as assessed by 12- h test-weighing, was not affected by the infection. Concentrations of milk total protein, casein, and whey proteins were similar in the two groups and there was no significant effect of illness on milk trace element concentrations. Thus, acute maternal infections during established lactation did not affect milk volume, milk protein, or trace element concentrations, despite expected changes in serum protein and trace element concentrations.