Introduction: Ascites is observed in cancer patients as well as in other non-neoplastic processes. In some patients, it may cause severe symptoms that can become directly life-threatening. The assessment of the degree of ascites seems useful in the determination of treatment effects as well as in the monitoring of fluid accumulation and early planning of decompression procedures. Aim: Determination of the clinical usefulness of a quantitative method of determining the degree of ascites, so-called Ascites Index. Material and methods: The Ascites Index is an ultrasonographic way of assessing the grade of ascites. The examination result is an index which is analogous to the amniotic fluid index determined in pregnant patients. The Ascites Index was determined in patients with ascites in the course of stage III–IV ovarian carcinoma (7 patients) and ovarian hyperstimulation syndrome (12 patients). Results: The patients with ovarian hyperstimulation syndrome required decompressive paracentesis at the median Ascites Index above 290 mm (range: 216–386 mm). In the patients with ovarian carcinoma, the median value of the Ascites Index at which paracentesis was required was 310 mm (range: 273–389 mm). To avoid complications associated with excessive protein loss, 2000 mL of fluid was evacuated at a single occasion. Following the procedure, the median value of the Ascites Index was 129 mm (range: 121–145 mm) in the patients with ovarian hyperstimulation syndrome and 146 cm (119–220 mm) in cancer patients. Conclusions: The proposed index is simple and rapid to determine. It makes evaluation of the degree of ascites considerably easier. Moreover, it only minimally burdens patients and enables assessment of the effect of decompression or treatment. It seems that this method might be useful also in the assessment of ascites caused by other factors, but this requires further clinical studies.