We have estimated how much of the total genetic predisposition to SLE may be attributable to genes outside the HLA region by comparing figures for concordance of SLE in monozygotic twins with those for concordance in HLA identical siblings in Australia. None of six dizygotic co-twins of white Australian SLE probands was concordant for SLE. One of four (25%) monozygotic co-twins of white Australian SLE probands was concordant for SLE which when added to previously published figures for Caucasoid populations gives an overall concordance rate for SLE in monozygotic twins of 25%. None of 18 HLA identical, same sex siblings of SLE probands, had definite SLE by the study criteria (i.e. less than 6%). The comparison of these figures shows that most of the genetic predisposition to SLE is attributable to genes outside the HLA region.