In existing query-based routing protocols in wireless sensor networks (WSNs), a node either keeps precise route information to desired events, such as in event flooding, or does not keep any route to desired events such as in query flooding. In this paper, we propose a routing protocol, called Hint-based Routing by Scope Decay Bloom Filter (HR-SDBF), that employs probabilistic hints. In HR-SDBF, each node maintains some probabilistic hints about events and utilizes these hints to route queries intelligently. We also put forward a data structure, Scope Decay Bloom Filter (SDBF) to encode the probabilistic hints.With SDBF, the amount of information about an event is propagated, without any loss, within the k-hop neighborhood of an event source but decreases outside the k-hop neighborhood as the distance from the event source increases. Compared to existing query-based protocols, HR-SDBF greatly reduces the amortized network traffic without compromising the query success rate and achieves a higher energy efficiency. To the best of our knowledge, this is the first query routing protocol in WSNs that utilizes probabilistic hints encoded in a variant of the bloom filter. Both the analytic and the experimental results support the performance improvement of our protocol.