OBJECTIVE: To investigate the effect of cohabitation status in older men and women on (a) onset of disability at 3- and 4.5-year follow-up and (b) changes in functional ability between 3- and 4.5-year follow-up, and to analyze whether this effect was mediated by social participation. METHOD: A total of 2,533 nondisabled older men and women enrolled in the Danish Intervention Study on Preventive Home Visits constituted the study population. Data were collected by mailed questionnaires in 1998-1999, 2000, 2001-2002, and 2003. RESULTS: Living alone significantly increased the risk of onset of disability (T3 OR = 1.60[1.06-2.43], T4 OR = 1.74[1.22-2.47]) and the risk of sustained poor functional ability (OR = 2.35[1.44-3.84]) among men, but not among single-living women. Social participation mediated only a small part of the effect of cohabitation status on functional ability. DISCUSSION: Our results underline the importance of cohabitation/marriage for maintaining a high functional ability among older men.