The purpose of this pilot study was to evaluate the acute gastrointestinal morbidity of adjuvant radiotherapy (RT) for Stage I seminoma of the testis. Ten Stage I patients receiving para-aortic and ipsilateral pelvic nodal (dog-leg) RT provided a toxicity baseline (group A). Twenty Stage I patients, randomized to dog-leg RT or para-aortic RT (10 per group) were further randomized to received prophylactic ondansetron or expectant therapy with metoclopramide (group B). Daily patient-completed questionnaires evaluated acute toxicity. In group A (n = 10), nausea, vomiting, diarrhoea and abdominal discomfort were experienced in 90%, 80%, 70% and 90% respectively. Antiemetic and antidiarrhoeal agents were required in 70% and 10% respectively, with good response. For group B (n = 20), the overall incidences of nausea, vomiting diarrhoea and abdominal discomfort were 80%, 45%, 60% and 80% respectively. The ondansetron group experienced less nausea (P = 0.02) and less vomiting (P = 0.06). Both reduced field size and ondansetron groups appeared to have less diarrhoea (P = 0.06). The use of antiemetics in the expectant therapy groups resulted in at least a two-level reduction of toxicity grade in 86% of patients. A high incidence of lethargy, anorexia and headaches was noted for all groups. The incidence of headaches was not increased with ondansetron. Dog-leg RT for Stage I seminomas is associated with readily demonstrable gastrointestinal tract (GIT) toxicity. The number of patients in this study is too small to produce definitive results, but there appears to be reduced GIT toxicity with prophylactic antiemetics. The effect of reduced RT fields has been assessed further in the MRC randomized trial of field sizes (TE10).