With nearly one quarter of mammalian species threatened, an accurate description of their distribution and conservation status is needed [1]. For rare, shy or cryptic species, existing monitoring methods are often prohibitively expensive or unreliable. The problem is particularly acute in tropical forests, where a disproportionate number of species are listed by IUCN as 'data deficient'[2], due to the difficulty of monitoring with conventional approaches. This presents serious obstacles to conservation management. We, here, describe a new screening tool, the analysis of mammalian DNA extracted from haematophagous leeches. By demonstrating that PCR amplifiable mammalian blood DNA survives for at least four months post feeding in haematophagous Hirudo spp. leeches, we hypothesise that most wild caught adult leeches will contain DNA traces of their last blood meal. We subsequently demonstrate the efficacy of the method, by testing it in situ using terrestrial Haemadipsa spp. leeches caught in a tropical Vietnamese rainforest setting, and identify cryptic, rare and newly discovered mammalian species. We propose that DNA from leeches represents a quick, cost-effective and standardised way to obtain basic data on mammalian biodiversity and species occupancy, facilitating efficient use of limited conservation resources.