Childcare arrangements are key in women's ability to juggle motherhood and work outside the home. As such the study of access to childcare and its use is of great policy relevance. We focus on a particular kind of informal childcare, the one provided by grandparents. Empirically, assessing the effect of grandparental childcare is not an easy task due to unobserved preferences. In light of the potential outcome framework, we interpret the biases resulting from unobserved preferences as arising from the non-compliance of mothers to the availability of grandparents and from preferences of grandparents for activities other than childcare. Using an instrumental variable approach on Italian data, we find that the effect of grandparental childcare on mothers' labour supply is positive, statistically significant and economically relevant. The effect is stronger for less educated mothers, with young children and living in northern and central Italy.