The proportion of the population treated for major medical conditions, including diabetes, cancer and mental illness, increased rapidly during the 1990s. We document the magnitude of these increases and use a model of prevalence to identify three potential causes: increased clinical incidence of disease, longer survival times among persons with chronic illnesses and increased detection. We present a series of analyses to evaluate the contribution of each factor. We find that increases in obesity explain a large proportion of the increase in treatment rates for conditions closely linked to obesity (e.g. diabetes). We provide some evidence that increases in treated prevalence unexplained by changes in the underlying clinical incidence of disease are driven by increased detection and treatment of patients with ‘subclinical’ illness.