The syndrome of adenocarcinoma of unknown primary (ACUP) is a frequent problem in both medical and surgical practice. The prognosis is poor, the median lifespan being 4 months. In general, multiple invasive procedures aimed at determining the primary tumour are not warranted due to the low frequency of detecting a tumour for which adequate treatment exists. In this paper we wish to highlight a subset of female patients presenting with malignant ascites and no evidence of a pelvic mass, who on laparotomy were found to have primary peritoneal papillary serous adenocarcinoma. These tumours must be regarded as a potentially treatable subset of patients with ACUP in view of their frequent response to chemotherapy and relatively good prognosis.