In 1992, the National Institute on Aging (NIA) and the National Cancer Institute (NCI) initiated a study to assess the prevalence of comorbid conditions in elderly patients with cancer. Seven cancer sites were selected for the study: breast, cervix, ovary, prostate, colon, stomach, and urinary bladder. This report on approximately 7600 patients in the study sample describes the NIA/NCI approach to developing information on comorbidity in elderly patients and addresses the chronic disease burden (i.e., comorbidity) and severity for six particular conditions: arthritis, chronic obstructive pulmonary disease (COPD), diabetes, gastrointestinal problems, heart-related conditions, and hypertension. Data on comorbidity were collected by abstracting information from hospital medical records. Patients were registered in six geographic areas of the NCI Surveillance, Epidemiology, and End Results (SEER) Program. A stratified random sample of patients aged 55 to 64, 65 to 74, and 75 years or older-with the index cancers were selected. Comorbidity data were matched with data from the conventional SEER monitoring system. Analyses showed that hypertension is the most prevalent condition and is also much more common as a current management problem rather than as history for the NIA/NCI SEER Study patients. Heart conditions varied slightly in the percentage of severity reported, but percentages for all tumors remained within a range of 13 to 26% for current and past categories. A similar range was observed for arthritis, with the higher percentage seen in the current problem category. For episodic complaints (e.g., gastrointestinal problems), a medical history was more common, except for cancers that involve complaints associated with the malignancy (e.g., colon and stomach cancers and, to a lesser extent, ovarian cancer). COPD and diabetes were less prevalent. Analyses currently under way will determine the impact of a patient's comorbidity burden on the cancer care continuum of diagnosis, treatment, and survival. The broad and independent effects of chronic conditions, singly and in combination, are being examined.