Phenobarbital, diazepam, lorazepam, and phenytoin are all currently used for the treatment of acute seizures, including status epilepticus. None of these drugs is considered ideal. Fosphenytoin is a new phenytoin prodrug that fulfills many of the properties of an ideal anticonvulsant drug. The safety, tolerance, and pharmacokinetics of intramuscularly administered fosphenytoin have been evaluated in three clinical trials involving patients requiring loading or maintenance doses of phenytoin. These investigations demonstrated that fosphenytoin is rapidly and completely absorbed after injection into muscle and is quickly converted to produce therapeutic phenytoin plasma concentrations within 30 min of administration. Plasma concentrations of phenytoin achieved with i.m. fosphenytoin exceeded those associated with an equimolar dose of oral phenytoin. i.m. fosphenytoin was well tolerated both locally and systemically. Only mild and transient reactions occurred at the injection site. The most common systemic adverse events reported--somnolence, nystagmus, dizziness, and ataxia--are side effects commonly seen with phenytoin and tended to be mild. Preexisting seizure disorders remained stable. Combination treatment with i.v. diazepam or lorazepam to attain rapid seizure control and i.m. fosphenytoin to maintain the anticonvulsant effect theoretically offers many advantages for control of acute seizures and should be studied.