A 35-year-old woman with a histologically proven T2 N0 M0 adenocarcinoma of the breast was given 4 cycles of neodjuvant chemotherapy then underwent tumourectomy followed by irradiation. Lymph nodes were free from invasion. A 7 cm ovarian cyst developed during follow-up and after a 2 month regimen of lynestrenol, coelioscopy with peritoneal lavage was performed. The pathology diagnosis was papillary cystadenocarcinoma requiring laparotomy which revealed invasion of both ovaries, neoplasic granulations and involvements of the epiloon. Exceresis of the trocar tract also showed tumoural invasion. Pathology examination favoured metastatic extension of the breast cancer. A 6-month chemotherapy was programmed before a second look. This case illustrates the risk of neoplastic dissemination after puncture or rupture of a cyst misdiagnosed as benign. In patients with a history of breast cancer, the risk of discovering a primary malignant ovarian cancer at coelioscopy is 9%. Metastases are found in 15-30% of the cases compared with 0.4 to 1.8% in unselected subjects. Prevention of operative dissemination relies on cystectomy without opening the cyst. This may require transforming the coelioscopy into a laparotomy which should not be considered as an operative failure but as a necessary method of preventing dissemination and clinical aggravation. Careful history taking, a rigorous coelioscopy technique and extension to laparotomy in cases with suspected diffusion should reduce the number of dramatic situations where the malignant process had not been suspected in the precoelioscopic diagnosis.