In unilateral kidney disease, it has been suggested that the ipsilateral state could be estimated by measuring contralateral kidney size; thus, a contralateral increase should reflect an ipsilateral impairment and vice versa. This could easily be performed using ultrasound. In our experiments with partial obstruction of one ureter (created in 1- to 3-day-old rats), a consistent and stable contralateral kidney hypertrophy occurred. However, after unobstruction, the hypertrophy persisted for at least a 6-week observation period, despite the disappearance of hydronephrosis and presence of a normal weight of the ipsilateral kidney. This would undeniably hamper the diagnostic utility of kidney size determination. To find out whether or not the hypertrophy ever disappears, we have performed studies after 6, 9 and 15 weeks. (1) The weight of the unobstructed kidney was found normal except after 15 weeks. The microstructure was normal except for some minor changes. (2) The contralateral kidney was significantly heavier than controls at all checkpoints (+10, +13 and +6%). No significant regression occurred over the observation period. (3) Furthermore, the weights of both kidneys were interrelated, but the slopes were positive; thus, the contralateral hypertrophy was not greater the lesser the ipsilateral kidney weight. The reasons for the persistence of contralateral hypertrophy are discussed. The present findings do not support the idea of documenting improvement by measuring contralateral kidney size. However, repeat studies in the same individual may prove to be more sensitive and are therefore recommended until further notice.