Lignac-Fanconi disease has often been described in siblings but never in different generations. Our findings in the families of eight patients, including chromatographic studies of 138 urine specimens, confirm these observations. Of a total of 19 siblings four were probably and one certainly affected. No other relatives suffered from the disease. Consanguinity was found in one family; its incidence also seems to be raised in the literature. The data suggest that Lignac-Fanconi disease is genetically determined and of simple Mendelian recessive character, but that it is not genetically related to classical cystinuria or to the so-called Fanconi syndrome in adults. The frequency of Lignac-Fanconi disease in the general population is estimated to be roughly 1 in 40,000 with a gene frequency of 1 in 200. The genetical prognosis is believed to be 1 in 4 for any subsequent child of parents who already have one sick child. Corresponding figures are given for cousins of the patient and for children of the patient's sibs.