A retrospective analysis of 54 patients diagnosed of neoplasia of unknown origin between January 1983 and December 1987 are presented. The patients's characteristics, histologic type, localization of metastasis, diagnostic procedures used and their cost-effectiveness, treatment, survival, and percentage of diagnosis of primary disease, are studied by clinical follow-up o necropsy studies. Our results are compared with those described in the literature. Adenocarcinoma was the predominant histologic type (48%). The cost-effectiveness of diagnostic tests was practically none in those performed without a clear clinical sign of suspicion, and very low in those performed with a clinical suspicion. The over all survival was 13 months with an actuarial survival probability at 24 months of 34%. We highlight the importance of an organized and predetermined clinical approach in this kind of patients, performing complementary tests only under the suspicion of a sign, or if a disease has to be discarded before initiating active oncologic treatment.