There is an increasing literature on the effect of grandparental childcare on the labor supply of adult daughters. However, the endogeneity of grandparental childcare remains an important empirical issue. This paper proposes a new quasi-experimental approach: We make use of a Norwegian retirement reform to estimate the causal effect of the grandmother’s retirement on the labor supply of her adult daughter. The preliminary findings show that grandmother’s retirement increases the employment and wages of her adult daughter. Further, grandparental childcare is the most likely channel of the effects as there are no effects of grandmothers’ retirement on the labor supply of adult daughters without young children or adult daughters living far from the grandmother.