Rubinstein–Taybi syndrome (RTS) is characterized by mental retardation, broad thumbs and great toes and a recognizable craniofacial phenotype. Causative mutations have been described in the CREBBP and EP300 genes. Here we present a 19‐year‐old woman and an unrelated 3‐year‐old boy, both with broad thumbs and halluces, but with facial aspects distinct from those of typical RTS. The woman had a marked learning disability, but no mental retardation. We identified a de novo c.7100delC mutation in EP300 (which predicts p.P2366RfsX35) in the woman and an apparently de novo c.638delG mutation in the boy, which predicts p.G213EfsX6. Mutations in EP300 are a known but rare cause of RTS. Only five other patients have been reported. We propose that individuals with EP300 mutations may exhibit a slightly different phenotype compared to individuals with CREBBP mutations, with milder cognitive impairment, more pronounced microcephaly, absent or mild downslanting of palpebral fissures, distinct arched eyebrows, and greater degree of retrognathia. © 2009 Wiley‐Liss, Inc.