Patients are given SU5416 via twice-weekly intravenous infusions in 4-week treatment cycles. Any patient who has not experienced unacceptable toxicity and who is deemed to be responding to the study drug (no evidence of disease progression) is permitted to continue receiving SU5416 in 4-week treatment cycles (up to a maximum of 1 year of therapy) until that patient experiences either unacceptable toxicity or tumor progression, as defined in the protocol.