Two unusual sulphur-containing amino acids have been isolated from urine of a baby who died with major physical malformations and failure of growth and development. Sensitive mass spectrometric methods were used to identify the nanomole quantities of the compounds available as S-(2-carboxypropyl)-cysteine and S-(2-carboxypropyl)-cysteamine. Incubation of fibroblasts in either [14C]Valine or [35S]cysteine resulted in radioactive labelling of the compounds, suggesting their origin from conjugation of methacrylic acid with cysteine and subsequent decarboxylation of the cysteine conjugate. Specific assay of methacrylyl-CoA hydratase is needed for final proof that this is a new inborn error of that enzyme, but these findings and parental consanguinity make this very likely. It seems possible that methacrylic acid or one of its derivatives may have caused the malformations present in the baby.