BACKGROUND: Nausea and vomiting associated with chemotherapy are common side effects which remain difficult to control. Acute phase nausea and vomiting (0-24 hours after induction of chemotherapy) parallels plasma serotonin release, which explains the effectiveness of 5-HT3 receptor antagonists. Serotonin released from gastrointestinal enterochromaffin cells may mediate chemotherapy-induced emesis. In this study, we analyzed urinary excretion of 5-HIAA, the main metabolite of serotonin. METHODS: Eight men and four women were studied in their cisplatin chemotherapy cycle. Urinary 5-hydroxyindoleacetic aicd (HIAA) levels were determined before and during a 24-hour period under ondansetron prophylaxis. RESULTS: Urinary 5-HIAA excretion for a 24-hour period was increased in all patients after induction of cisplatin (P=0.002). CONCLUSION: Cisplatin chemotherapy is associated with serotonin release in the acute phase. Our finding provides evidence for a relationship between emesis and serotonin following cisplatin chemotherapy.