Various factors possibly influencing responsiveness to thyrotropin-releasing hormone (TRH) therapy were studied in 38 children (20 M, 18 F) with severe epilepsy. Mean age at treatment was 4.7 years (range; 0-18 years). Seizure type was infantile spasms (IS) in 16, generalized tonic seizures in 8, secondarily generalized partial seizures in 4, generalized tonic-clonic seizures in 2, atypical absence in 5, myoclonic seizures in 1, and atonic seizures in 2 cases. All seizure types were classified by ictal EEGs documented by simultaneous EEG-VTR according to the International Classification of seizures, except for two with atonic seizures and one with IS. Factors analyzed were sex, age, etiology, neurologic abnormality, seizure types, seizure frequency, EEGs, duration of TRH therapy, and serum hormone [human growth hormone (HGH), prolactin (PRL), thyroid-stimulating hormone (TSH)] levels before and after TRH therapy. We showed that: (a) serum PRL level was significantly correlated to effectiveness of TRH therapy (the higher the PRL, the greater the response rate); (b) serum basal PRL decreased significantly, especially in good responders; and (c) serum basal PRL was elevated especially in patients with IS, which responded more to TRH therapy. These interesting findings seen in severe epilepsy of childhood deserve further neuroendocrinologic study.