This study looked at whether toddlers posit the existence of unobserved causes when events occur probabilistically. Older (18-24 months) and younger (12-17 months) children were introduced to novel events. An experimenter pressed a red handle and a lollipop emerged from a box; she then pressed a green handle and a cake emerged. These events were repeated three times. On the fourth trial, the experimenter switched either the order or relationship between events. In the Deterministic condition, the experimenter pressed the green handle first and the red handle second; in the Probabilistic condition, the red handle produced the cake and the green handle produced the lollipop. On the test trial, the experimenter pressed the red handle and a hand emerged, holding the lollipop. The older toddlers looked longer at the hand in the Deterministic than the Probabilistic condition, suggesting they inferred a hidden cause when the events occurred probabilistically.