OBJECTIVE: Autoimmune polyglandular syndrome type 1 (APS-1) is a disease associated with defects of the autoimmune regulator gene and is characterized by autoimmune lesions of several tissues, predominantly endocrine glands, with multiple autoantibodies. In this study we describe autoantigenic epitopes on cholesterol side-chain cleavage enzyme (P450scc) using sera from Finnish and Sardinian patients with APS-1, and analyze the epitope reactivities during disease follow-up. METHODS: A series of P450scc cDNA fragments were expressed in E. coli and tested by immunoblotting assay using the patients' sera. RESULTS: Epitope regions were found over the whole P450scc molecule except the last N- (amino acids (aa) 1-40) and C-termini (aa 456-521). The strongest reactivity with patients' sera was found with central and C-terminal regions of the P450scc protein. All studied patients had IgG1 subclass antibodies. CONCLUSIONS: The results show that Finnish and Sardinian patients with APS-1 have similar, polyclonal immune reactions against P450scc, and that epitope reactivities did not change during the disease course. These results support the opinion that autoantibodies against P450scc and their epitope reactivity pattern are formed at an early stage of steroidogenic autoimmunity.