A population-based study of functional status and social support networks of elderly patients newly diagnosed with cancer. We assessed the functional status and social support networks of 799 men and women aged 65 years or older newly diagnosed with cancer and living in six New Mexico counties. Functional limitations included depending on others for transportation (33%) and mental incompetence or poor recent memory (42%). The percentage of patients with functional limitation increased sharply with increasing age. In a substantial number of patients there was also evidence for poor social support networks; 26.5% of subjects lived alone and 38.9% had no children living in the vicinity. In a multiple logistic regression analysis, the predictors of having a poor social support network included non-Hispanic white ethnicity, advanced age, low income, and being a recent migrant to the area. Subjects with functional limitations were more likely to have poor social support networks than subjects without such limitations. The deleterious combination of impaired functional status and a limited social support network may explain why elderly cancer patients are at increased risk for not receiving appropriate therapy. Given the potential complexities involving the evaluation and appropriate treatment of cancer, care must be taken to adequately assess functional status and support mechanisms of older patients, and to provide adequate support to ensure compliance with treatment. 