A subgroup of autoimmune type chronic active hepatitis (AI-CAH) is associated with LKM-1 autoantibodies. They are directed against Cytochrome P450 dbl and inhibit its function in vitro. We observed LKM-1 positive AI-CAH and colitis in a 13 year old girl. Liver disease responded well to corticosteroids. Father, mother, brother, the patient and her identical twin sister were investigated for HLA class I–III phenotypes, auto-antibodies, in vitro inhibition of P450 dbl, and in vivo phenotype for drug metabolism (sparteine) mediated by this enzyme. Both twins had the autoimmune HLA haplotype A1, B8, DR3, C4A-Q0. While the mother is a homozygous extensive metabolizer (EM) (metabolic ratio 0.33), the father is a homozygous poor metabolizer (MR 65.25) and all children are heterozygote EM (MR 1.01, 0.99 & 1.76). Only the index patient had signs of liver disease and was positive for LKM-1 antibodies, and only her serum inhibited P450 dbl catalysed oxidation of sparteine in vitro up to 90 %.Conclusion: We describe for the first time occurence of LKM-1 positive AI-CAH in a pair of identical twins, who were discordant for the disease. Since both twins are of the EM metabolizer type, i.e. express functionally intact P450 dbl, and share the autoimmune HLA haplotype, we conclude that environmental factors trigger the manifestation of this autoimmune liver disease.