Serum pipecolic acid was measured in patients with neonatal adrenoleukodystrophy (NALD), sex-linked ALD, and the cerebrohepatorenal syndrome of Zellweger. Pipecolic acid was elevated, often markedly, in most of the patients with NALD but in none of those with X-linked ALD or adrenomyeloneuropathy, or in normal adults and children, or children with cirrhosis or other neurodegenerative disorders. The demonstration of elevated serum pipecolic acid in NALD adds to the previously recognized elevation of very-long-chain fatty acids (another Zellweger syndrome sign) and suggests that generalized peroxisomal dysfunction may be a characteristic of NALD as it appears to be of Zellweger syndrome, which diseases share some clinical characteristics. The nosologic significance of these findings is discussed.