I n our thr ee-stage questi onnai r e study w e i nvesti gated patter ns of tw i n and familial aggregation of osteoar thr i ti s (OA ) for commonl y affected joi nts. The basel i ne questi onnai r e study of the Fi nni sh Tw i n Cohor t w as per for med i n 1975. I n 1990, 4095 tw i n pai r s of the same gender bor n 1930-1957 r esponded to a questi onnai r e and r epor ted w hether they had OA di agnosed by a physi ci an. I n 1996 both tw i ns of 266 pai r s of w hi ch at l east one had r

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... ted OA i n 1990 r esponded to a detai l ed questi onnai r e on joi nt-speci fi c OA , i ncl udi ng fami l y hi stor y of OA . I n mal e pai r s shar ed (nongenetic) familial effects accounted for 37% of the total var i ance i n l i ability to OA and unshared envi r onmental effects for 63%. I n femal e pai r s addi ti ve gene effects expl ai ned 44% of the var i ance in liability to OA, and unshared environmental effects for 36%. Familial aggregation of finger and k nee OA w as cl ear l y hi gher than that of hi p OA . Tw i n-pai r di scor dance for OA w as, to some extent, associ ated w i th body-mass i ndex, occupati onal l oadi ng and tr auma. Our r esul ts i ndi cate that geneti c effects may be modul ated by sex or by envi r onmental factor s di str i buted di ffer entl y betw een men and w omen. Based on our joi nt-speci fi c data fi nger and k nee joi nts ar e the most opti mal tar gets for studi es of geneti c factor s pr edi sposi ng to the devel opment of OA . Keyw or ds: osteoarthri ti s, epi demi ol ogy, geneti cs, tw i n research Correspondence: Dr UM Kujal a, Uni t for Sports and Exerci se M edi ci ne, M annerhei mi nti e 17 (Töölö Sports Hal l ), FIN-00250 Hel si nki , Fi nl and.