Double-blind, placebo-controlled, cross-over trial of progabide as add-on therapy in epileptic patients. In a double-blind, cross-over trial, progabide (PGB) and placebo were compared as add-on therapy in 59 patients with moderate to severe epilepsy. Eight patients did not complete the study, 4 because of adverse drug reactions (elevation of liver transaminases, 2; gastritis, 1; and acute psychosis, 1) and 4 because of administrative reasons. Among the remaining 51 patients, seizure frequency was reduced greater than 50% in 18 patients with PGB treatment and in 8 patients with placebo (p less than 0.05). The number of days with seizures was significantly (p = 0.034) reduced during PGB treatment. Both patients' and physicians' preferences at the end of the trial were in favor (p less than 0.01) of PGB. Mild clinical side effects were present in 54.7% of the patients treated with PGB and in 37.7% with placebo. Increase in liver transaminases was observed in 2 patients during the double-blind study and in 1 during the follow-up period. Our data show that PGB, as previously reported, is useful in 30-40% of patients who are not responding completely to other antiepileptic drugs (AEDs). The compound is well tolerated, but liver function must be monitored. 