This paper presents a theoretical and empirical model of child care and the labor supply of married women. Unlike previous studies, we treat child care costs as a mother's fixed cost associated with entry into the labor market. According to Cogan (1981), an implication of the existence of entry costs is that individuals will not be willing to work below reservation hours. A maximum likelihood estimator that allows reservation hours to be nonzero and differ randomly among individuals is developed. Using data from the 1993 Survey of Fertility and Employment of Married Women in the Taiwan Area, our estimation results reveal that the entry cost model is superior to conventional models in predicting hours of work and decisions as to whether one workds and whether one uses market care.