Longitudinal studies of the nutritional status of 60 lactating Bangladeshi mothers from an underprivileged, periurban community and of the quantity and composition of their milk were completed to determine the relationships between maternal nutritional status and lactational capacity. Daily milk production was estimated by 24-hour test-weighing; the nitrogen, fat, lactose, and total energy concentrations of extracted milk samples were analyzed at various stages of lactation to estimate total milk nutrient production. Although the mothers were poorly nourished compared with international reference populations, their lactational capacity was not severely impaired. Average milk production peaked at 750 g/d when the infants were between 5 and 7 months of age. Nitrogen and fat concentrations declined with (log) infant age; lactose concentration increased with (log) infant age (P less than .001). Average concentrations of milk nutrients when the infants were 3 months of age were: nitrogen, 0.161 g/dL; fat, 2.804 g/dL; lactose, 7.92 g/dL; energy, 61.0 kcal/dL. Fat and energy concentrations were significantly greater, and fat and energy amounts tended to be greater, for mothers with larger triceps skinfold thickness or arm circumference. Changes in nutritional status within individual women were also significantly related to the amount and composition of their milk: within-woman increases in triceps skinfold thickness were associated with increases in fat and energy concentrations (P less than .01) and within-woman increases in body weight were associated with increases in the amounts of milk and all major nutrients (P less than .01). Milk production declined significantly during certain months of the year, just before the major harvest period. The findings suggest that, despite their remarkably good lactational capacity, the mothers' milk production was limited to some extent by their nutritional status and may, therefore, be further increased with nutritional improvement.