Chromosome analysis in 13 patients with true Klinefelter's syndrome revealed in the majority of the cases a variation of the typical number of 47 chromosomes characteristic for the disease. The deviation varied in positive or negative direction or both, that is, in addition to the typical complement of 47 chromosomes the number ranged between 45 and 49. Clinically there was a pronounced correlation between the pathologic karyotypes and the low intelligence of the patients.