Aims: In this study, we investigated whether free intraperitoneal tumour cells (FITC) were detectable in ascites or peritoneal lavage fluid (PLF) from patients with peritoneal metastasis (PM) before and after treatment with pressurised intraperitoneal aerosol chemotherapy (PIPAC). Methods: Ascites or PLF retrieved at the first and third PIPAC procedures was analysed by conventional cytology, carcinoembryonic antigen (CEA) and total protein concentration, and quantitative reverse transcriptase PCR (qRT-PCR) for mRNA expression of CEA, epithelial cell adhesion molecule (EpCAM) and cancer antigen 125 (CA-125). Conventional cytology and qRT-PCR were also performed in a negative control group (benign PLF specimens and inflammatory ascites). The treatment response was compared with the histological response based on repeated peritoneal biopsies evaluated by the Peritoneal Regression Grading Score (PRGS). Results: Thirty-five patients with PM of various origins were included from 2015 to 2016. At the first PIPAC procedure, FITC were detected by conventional cytology (sensitivity 0.58, specificity 1.00), CEA protein (cut-off 0.4 μg/L, sensitivity 0.71), CEA mRNA (sensitivity 0.75, specificity 1.00), EpCAM mRNA (sensitivity 0.71, specificity 1.00) and CA-125 mRNA (sensitivity 0.43, specificity 1.00). The combination of CEA/EpCAM mRNA had a sensitivity of 0.88 and a specificity of 1.00. The evaluation of ascites or PLF retrieved at the third PIPAC procedure failed to detect treatment response, when compared with the histological PRGS. Conclusions: The evaluation of CEA and EpCAM mRNA detects FITC with a high sensitivity and an excellent specificity, but is not useful for response evaluation in patients treated with PIPAC. Trial registration number: NCT02320448.