Some people with phenylketonuria who were born before screening began were never treated and are still alive. Here we report that far fewer people with untreated phenylketonuria were detected than are thought to exist (about 2000). The majority of those traced had high support needs, challenging behaviour and other symptoms of phenylketonuria. No significant differences were found between those who had or had not tried the phenylalanine-restricted diet. A randomised controlled trial is required to examine the effect of trying the low-phenylalanine diet for people with untreated phenylketonuria.