Background: Mental health problems are known to be hard to recognize in people with intellectual disabilities. One factor that may contribute to this is the diagnostic overshadowing bias, which describes the tendency of the clinicians to overlook symptoms of mental health problems in this client group and attribute them to being part of "having an intellectual disability". Although a small amount of research has investigated this in the USA, very little has taken place in the UK. Materials and methods: Two clinical vignettes were produced. Both described identical clinical problems, except that one vignette described a man with an IQ of 58 and impaired social functioning (i.e. an intellectual disability) and one a man with an IQ of 108 and normal social functioning (i.e. non-intellectually impaired). Psychiatrists (n = 274) and clinical psychologists (n = 412) throughout UK were randomly assigned to either the low IQ or normal IQ condition, and sent a corresponding clinical vignette. Results: One hundred and thirty-three psychologists and 90 psychiatrists responded. Diagnostic overshadowing did appear to be present. Clinicians were more likely to recognize a range of symptoms in those with IQs in the normal range than those with intellectual disabilities. Furthermore, psychiatrists appeared more likely to make this error under some circumstances than clinical psychologists. Conclusions: Diagnostic overshadowing may contribute to the difficulties that mental health professionals commonly experience in identifying mental health problems in people with intellectual disabilities. However, the methodology commonly used in this type of research has a number of weaknesses, and would benefit from an alternative approach.