Chemotherapy-induced emesis is one of the major problems in the treatment of oncologic patients. Recently, a novel class of compounds, the selective 5HT3 receptor antagonists, has been introduced, achieving a dramatic improvement in the control of emesis. The absence of extrapyramidal side effects adds to their safety and good tolerability. The Authors herein analyse their experience on 269 cycles of chemotherapy in 47 patients treated for gynaecological and breast malignancies, with particular regard to adverse events such as headache. Their most frequent side-effects are headache and constipation, that are usually mild and self-limiting. Nevertheless, in some cases, severe, rebel headache has been reported, leading in our experience in 6.4% of cases to discontinuation of the antiemetic regimen. A previous history of recurrent or severe headache or migraine is not correlated with the occurrence of ondansetron-induced headache, as severe headache occurred after ondansetron only in 28.4% of the patients with positive anamnesis, and 70% of the patients that experienced had never suffered from severe headache before. In those patients complaining of severe headache, the Authors suggest an antiemetic association, with a loading dose of ondansetron i.v., followed by metoclopramide i.m. orally for the following days.