Six large families with classical Marfan syndrome were studied using markers on chromosomes 1 and 11. Two of three families tested showed negative scores using D1S7 but a third family gave a positive score (0.92) at theta = 0.1. The other chromosome 1 markers typed (MUCI, NGFB, D1S8) excluded close linkage. Negative lod scores with two chromosome 11q22 markers (D11S84, D11S148) excluded at least 20 cM in this area (Z = less than -2), which was chosen for study as two enzymes responsible for collagen degradation (collagenase and stromelysin) are localised to this region.