X-linked adrenoleukodystrophy (X-ALD) is due to an impairment in the peroxisomal beta-oxidation of very long straight chain fatty acids (VLCFAs) and the gene involved encodes a 75 kD protein (ALDP). Prenatal diagnosis is usually made by measurement of VLCFAs in cultured amniotic fluid cells (CAF) and chorionic villus cells (CCV), but some misdiagnoses have been reported. For this reason, some authors suggest the use of more than one strategy to minimize the risk of pitfalls. In this study we show, by immunochemical techniques, that ALDP is expressed in chorionic villi and amniotic cells and can be used for prenatal diagnosis of X-ALD in kindreds where ALDP is absent (69-84 per cent), together with VLCFA determination. Moreover, we demonstrate that the culture medium modifies ALDP expression; therefore, it is a factor that must be taken into account when a prenatal diagnosis is done.