This paper examined associations between mothers' work schedules and children's cognitive outcomes in the first 3 years of life for approximately 900 children from the National Institute of Child Health and Human Development Study of Early Child Care. Both the timing and duration of maternal nonstandard work schedules were examined. Although results varied across developmental stages and dimensions of cognitive performance, the effects of mothers working nonstandard schedules tended to be negative, particularly if these schedules began in the 1st year of life, and particularly for measures of cognitive development at 24 months and expressive language at 36 months. Descriptive statistics suggested that these negative effects might be due to the type of child care used. Avenues for future research are discussed.