Compensatory renal enlargement was assessed in 19 adult patients who either had a nephrectomy (17 cases) or developed a functionless kidney following obstruction (two cases). Hypertrophy of the healthy kidney was assessed by comparing renal size on urography before and after removal or destruction of the diseased kidney. Compensatory renal enlargement was demonstrated in 40 per cent of the patients, including two patients in their sixties. The average increase in length was 3 per cent and the maximum increase in length was 9 per cent. Therefore compensatory enlargement does occur in adult life, but is much less than that occurring in childhood. The presence of a hypertrophied adult kidney over 17 cm in length usually indicates that the contralateral renal disease was present in childhood.