A symposium in honour of the centenary of the birth of Lionel Penrose, Galton Professor at UCL and editor of this journal from 1945 to 1965, was held on 13 March 1998 and attended by about 150 people. This included a number of distinguished scientists from several countries (particularly the USA) who returned to honour their mentor. The scientific theme of the day was the current understanding of genetic causes of mental handicap, a field in which Penrose made many major contributions. The programme is reproduced here. A detailed account of the meeting, including a summary of the papers presented is available from the Centre for Human Genetics, University College London, Wolfson House, 4 Stephenson Way, London, NW1 2HE.

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From a vast selection of seminal papers by Lionel Penrose, we have chosen to reprint his inaugural lecture on Phenylketonuria delivered at University College on 21 January 1946. This was published in the Lancet (29 June 1946, pp. 949–953) and is reproduced by kind permission.

We feel that in 1998 it is well worth re-reading!