In a doubleblind cross-over placebo-controlled trial the efficiency of oral treatment with branched chain amino acids was investigated in 22 inpatients with liver cirrhosis. In all patients evidence of latent (subclinical) portalsystemic encephalopathy was obtained by using an extensive psychometric test programme. Patients received a defined diet of 35 cal/kg/day containing 1 g of protein. In addition, branched chain amino acids or casein in a dosage of 0.25 g/kg/day was administered in a cross-over fashion, each for 1 week. Semiquantitative nitrogen balance increased during both treatments, with a tendency towards a larger increase during branched chain amino acid treatment. At the same time ammonia concentration tended to decrease during branched chain amino acid treatment. Taking into account the cross-over design, significant improvements attributable to branched chain amino acid treatment could be demonstrated in psychomotor functions (line tracing, tapping, steadiness, auditory reaction time), attention (digit table), and practical intelligence (digit symbol, number connection test).