Experience with first trimester diagnosis of inherited metabolic disease is still limited. In this report, data are collected from four major centres in the UK which provide a prenatal diagnosis service based on specific enzyme or gene product assay. The data were presented at a workshop on 'First Trimester Diagnosis of Inherited Metabolic Disease' held at the Institute of Child Health, London, on 21st June 1990. Approximately 100 different metabolic diseases can now be diagnosed in the first trimester, but because of the rarity of many of the disorders, experience of positive diagnoses, based on enzyme deficiency in fresh chorionic villus samples (CVS), cultured villus cells or early amniocentesis samples, is likely to be limited. It is, however, important that these results are reported and similarly that any problems which arise are fully documented.