Between 2% and 9% of patients with cancer present with metastatic nonsquamous cell carcinoma of unknown primary origin. Traditionally, a series of investigations is undertaken to locate the primary origin of the tumour, although many of these tests are often painful or distressing to patients, unsuccessful in locating the primary site and costly to the health care system. Moreover, even if a tumour is found it usually cannot be treated surgically. However, a small number of cancers of unknown primary origin can be cured, arrested or effectively palliated with systemic treatment. This study compares the costs and outcomes of the current practice of comprehensively searching for the primary tumour with those of an alternative, limited approach that identifies only the primary tumours for which relatively effective systemic therapy exists. Decision trees were constructed for the two diagnostic approaches and their associated therapeutic options. Costs and probabilities were integrated with published data on the survival of patients with each type of cancer. The results indicate that the comprehensive diagnostic strategy may increase 1-year survival rates from 11.0% to 11.5%. On the basis of Ontario cost data it is calculated that the additional costs of a comprehensive search for 1000 patients will range from approximately $2 million to $8 million, depending on the subsequent treatment strategy.