Criterion-related validity of a new measure of functional ability was conducted according to a causal model based on conceptual models employed in the area of rehabilitative and geriatric medicine. The criteria variables included concurrent diagnosed diseases, global self-rated health, drug consumption and general practitioner (GP) consultations. The measure of functional ability was developed with the intention of achieving a high degree of discrimination among a group of community dwelling elderly. Data were derived from a sample survey of 70-year-old men and women conducted in 1984 in the county of Copenhagen (Denmark). Altogether 366 men and 368 women participated in each of the two phases of the study--a comprehensive medical examination at the county hospital at Glostrup followed by a home visit conducted by an occupational therapist 1-2 weeks later. The analysis included four different unidimensional index scales of functional ability divided into two types, with reduced speed and tiredness as subdimensions. The two scale types were mobility function and lower limb function. Early losses of ability together with global self-rated health were treated as outcome measures of diagnosed chronic diseases. At the same time these outcome measures together with diagnosed diseases were considered to predict drug consumption and GP consultations. It was shown that functional ability as measured by the new index scales were strongly influenced by diagnosed diseases: arteriostenosis and osteoarthrosis in lower extremities, obesity, shoulder impairments and bronchitis among women, and glucose intolerance, arteriostenosis in lower extremities and shoulder impairments among men. Global self-rated health was strongly associated with the new functional ability rating system. Early losses of ability but not self-rated health was a strong predictor for drug consumption and frequent contacts with GP. It is concluded that the new measure of functional ability is suitable for health studies of community dwelling elderly, in particular as a summary statement of the individual's health status.