To determine how often disability in essential activities of daily living develops insidiously, and to evaluate whether the likelihood of insidious disability differs on the basis of physical frailty.

We conducted a prospective study of 754 nondisabled, community-living persons aged 70 years or older. Participants were categorized according to the presence or absence of physical frailty, which was defined on the basis of gait speed. Participants were subsequently followed with monthly telephone interviews for 3 years to determine the occurrence of disability in bathing, dressing, walking, or transferring, and to ascertain exposure to precipitating events, which included acute hospital admissions and other illnesses, injuries, or problems leading to restricted activity.

For first episodes of disability, 73 (36%) of 203 developed insidiously among the 322 participants who were physically frail and 26 (18%) of 141 developed insidiously among the 432 participants who were not physically frail (P <0.001). Physical frailty was the only factor that was associated significantly with the development of insidious disability, with an adjusted odds ratio of 2.4 (95% confidence interval: 1.4 to 4.1). The likelihood that an episode of disability was insidious increased progressively, from 29% of the 344 first episodes to 65% of the 155 fourth or higher episodes (P for trend <0.001).

Disability in essential activities of daily living often occurs insidiously, particularly among older persons who are physically frail or who have had prior episodes of disability.