Introduction In recurrent malignant glioma, early imaging response to two courses of chemotherapy is generally considered to be predictive of good survival. We studied the relationships between initial tumour volume and speed of imaging response to chemotherapy in malignant glioma. Methods In 43 chemotherapy naive patients, 26 glioblastoma multiforme (GBM) + 17 anaplastic astrocytoma (AA), median age 45 years, MRI responses to intravenous cereport and carboplatin were assessed at baseline, then at 2 monthly intervals. Patients were classified as fast responders if they had reached a partial response (PR) after two courses, and slow responders if PR was achieved after three or more courses of chemotherapy. Results PR occurred in four patients with GBM (15%) and nine patients with AA (53%). Likelihood of response was related to initial tumour enhancing volume in GBM but not in AA. PR occurred in four of five GBM patients (80%) with initial volume < 15,000 mm(3) and none of the 21 cases with an initial volume > 15,000 mm(3). In patients achieving a PR, there was no association between speed or duration of response and eventual survival. Fast responders with AA were significantly older than slow responders (p = 0.033). Conclusions Initial enhancing volume of GBMs may be an important predictor of imaging response. This has implications where response rates of phase II studies are reported and in stratification for phase III trials. Further work is necessary to confirm these findings with other types of chemotherapy and examine the relationship between proliferation markers and speed of response.