Peroxisomal disorders are genetic disease caused either by a defect in peroxisome biogenesis resulting in deficiencies of multiple peroxisomal enzymes or by a deficiency of one peroxisomal enzyme. Seventeen different peroxisomal disorders are recognized nowadays, with progressive neurological deficits and early death in most. Both in peroxisome deficiency disorders(PDD) like Zellweger syndrome, neonatal Adrenoleukodystrophy (n-ALD) and infantile Refsum disease, in classic rhizomelic Chondrodysplasia punctata(RCDP) and disorders with a defect in peroxisomal beta-oxidation of fatty acids (including X-linked ALD) no effective treatment is available and early prenatal detection indicated. For PDD this is done by us by in vitro measurement of acylCoA:dihydroxyacetonephosphate acyltransferase(DHAPAT) activity and immuno(western)blotting with antisera raised against catalase, acylCoA oxidase and peroxisomal thiolase(Thio) resp. in uncultured chorionic villi(CV) homogenates. Since 1992 seventeen affected PDS fetusses were recognized by us by this procedure in sixty one at risk pregnancies in the 10th or 11th gestational week. Moreover, an additional thirteen affected PDS fetusses were found by analyzing de novo plasmalogen biosynthesis and very longchain fatty acids(VLCFA) in cultured CV-fibroblasts or amniocytes. Seven RCDP fetusses were diagnosed by the finding of abnormal immunoblots with anti-thiolase (absence of 41kDa band for mature thiolase protein) and deficient plasmalogens in CV. We also diagnosed isolated DHAPAT deficiency in a fetus by finding a deficiency of plasmalogens and DHAPAT, but normal Thio-blot in CV. Fifteen fetusses with a deficiency of one of the enzymes of peroxisomal beta-oxidation, including X-linked ALD, were diagnosed prenatally in 41 at risk pregnancies by the finding of elevated VLCFA and deficient activity of peroxisomal beta-oxidation in cultured CV-fibroblasts or amniocytes. No false positive or negative cases were found with these procedures.