High-quality instruction in preschool is important for children's mathematical development. To date, the domain-specific elements constituting mathematics instruction quality and the factors associated with this quality are hardly studied, resulting in serious gaps in our insights into the topic. We aimed to address this gap by investigating (a) 43 preschool teachers' mathematical questioning behavior during shared picture book reading, and (b) its association with picture book and teacher characteristics. We analyzed the number and the level of abstraction of mathematical questions when sharing a mathematical (i.e., written with explicit mathematical aim and content) and a non-mathematical (i.e., written without explicit mathematical aim and content) picture book, and their associations with teachers' professional competence. For mathematical and non-mathematical picture books, teachers formulated mathematical questions, mainly at lower levels of abstraction. They formulated more, but proportionally less abstract, mathematical questions when sharing a mathematical versus a non-mathematical picture book. We found only limited evidence for associations between teachers' questioning behavior and their professional competence. Our findings indicate that preschool teachers offer opportunities to engage in mathematical interaction during shared picture book reading, pointing to the potential of this activity to implement high-quality mathematics instruction. They further show that mathematical picture books enable more mathematical interaction than non-mathematical picture books, but not at higher levels of abstraction. Future studies are needed to replicate and complement our findings, with special attention for the contribution of teacher professional competence and the fit to children's competency level.