Catabolism of histidine was investigated in 24 patients with different speech and language disorders and with significantly low histidase activity in stratum corneum. No classical histidinemia was found. Biochemical investigation of these patients after loading with L-histidine led to the conclusions that low histidase activity in stratum corneum was connected with: disturbances in folic acid metabolism (2 cases); "atypical histidinemia" (1 case); heterozygotes of histidinemia (2 cases); normal liver histidine metabolism but abnormal in other tissues (18 cases); previously unknown error of histidine metabolism (1 case).