X-linked adrenoleukodystrophy (X-ALD) (McKusick 300100) is a relatively common disorder due to activity deficiency of a peroxisomal transporter of very long chain acyl-CoA synthetase it causes accumulation of endogenous and exogenous saturated very long chain fatty acids (VLCFA) in plasma and in all tissues. X-ALD is characterised by phenotypic variability, about 80% of patients present more or less progressive demyelinization and adrenal insufficiency. The remaining patients have isolated adrenocortical insufficiency or are asymptomatic or presymptomatic. The measurement of VLCFA accumulation in plasma using the GC-MS method is the basis of diagnosis of X-ALD. Among 162 plasma samples from patients suspected of X-ALD, only 2 were from patients suspected of adrenomyeloneuropathy (AMN). In the analysed material we identified 13 X-ALD hemizygotes and 7 X-ALD heterozygotes. Twelve patients presented the childhood cerebral form of X-ALD. VLCFA profiles in plasma were analysed in 6 patients who were on a diet and Lorenzo oil.