Adult Polycystic Kidney Disease is an autosomal dominant condition and one of the commonest Mendelian diseases in the Caucasian population. The gene is said to be carried by about 1:1000 individuals. The principal pathological event characterising the disease is the development of multiple fluid filled renal cysts which increase in size and number and which may result in end stage renal failure. In 1985 a linkage was found between markers on chromosome 16p13.3 at the HBA locus and APKD. The locus was named PKD1. This opened the way for a genetic test for the condition as an aid to genetic counselling. This work consists of a study of 150 families ascertained through a renal clinic and followed up by a voluntary family register. This resource was used to measure the probability of detection of the APKD gene in the first degree relatives of probands with and without the aid of ultrasonographic imaging of renal cysts. In addition a study was made of the diagnostic efficiency of ultrasonography as a method of detecting gene carriers. Family studies with DNA genetic markers permitted a calculation of statistical risks for individuals from APKD families requesting genetic counselling. Over the period of this work the proportion of families and individuals informative for genetic marker studies improved as new markers were brought into diagnostic use. Use of DNA markers allowed the detection of families not transmitting APKD at 16p 13.3. Calculations were made of the probability of non PKD1 inheritance in individual families and of the rate of non PKD1 disease in the ascertained population. Finally a hypothesis was tested that linkage disequilibrium exists between close markers and the major (PKD1) locus.