The nausea and vomiting experienced by one in four cancer patients in anticipation of chemotherapy is probably a learned response to treatment. To determine whether behavioral approaches for altering learned responses might be useful treatments for these symptoms, we compared the effects of "systematic desensitization" (a behavioral treatment in which relaxation is learned as a response to situations in which patients have had anticipatory nausea and vomiting) with those of counseling and of no treatment. Sixty ambulatory cancer patients with anticipatory nausea and vomiting before their third and fourth chemotherapy treatments were randomized equally to the three groups. Significantly more patients receiving desensitization reported no anticipatory nausea before their fifth and sixth chemotherapy treatments than patients given counseling (P less than 0.05) or no treatment (P less than 0.01). Desensitized patients also reported significantly less severe anticipatory nausea (P less than 0.01) and vomiting (P less than 0.05) and a shorter duration of anticipatory nausea (P less than 0.01). We conclude that systematic desensitization appears to have an antiemetic effect in cancer patients who receive chemotherapy, and may be useful in the management of these problems.