Two cases of Hartnup disease were diagnosed in a five member family. A changeable polymorph and severe clinical features of a 16 year old girl was described. Total plasma amino acids value was significantly decreased in the girl compared to the sum of plasma amino acids value in the brother, mother, father and to the summed maximal values of normal range. Intermediate aminoaciduria was also found with atypical amino acids pattern. Total plasma amino acids concentration was significantly reduced (27.20%) in the mother, while no significant decrease in the son (1.83%) and father (7.51%) were found compared to the summed maximal values of normal range. In the clinicaly healthy father, 38 years of age, a gross aminoaciduria with atypical pattern of amino acids was also found. Urinary amino acids concentration in the son and his mother were rather normal, although low concentration of eight amino acids was found in the mother's urine. Cerebrospinal fluid 5-hydroxyindoleacetic acid level was reduced in the girl.