We present two patients with colorectal carcinoma who were admitted with fever and diarrhoea during treatment with chemotherapy. Blood cultures taken from both patients revealed an infection with Listeria monocytogenes. A contaminated ice cream was probably the source of infection in one patient. The other patient died of the listeriosis. Patients with diminished cellular immunity are at risk of invasive listeriosis. Listeriosis has a high mortality rate in this group of patients. Intravenous penicillin or ampicillin for 4 to 6 weeks is the first choice of antibiotic treatment. These antibiotics can be combined with an aminoglycoside or trimethoprim-sulfamethoxazole for a synergistic effect. To prevent invasive listeriosis we recommend strict dietary advice to all patients with diminished cellular immunity.