We hypothesized that highly variable cyst fluid sodium concentrations are a characteristic of every kidney in autosomal dominant polycystic kidney disease (ADPKD). We added our data on sodium concentrations in 124 fluids from ten ADPKD kidneys to data published by others of concentrations in 32 fluids from five kidneys. The values ranged from 3 to 207 mEq/liter; none fell between 59 and 74 mEq/liter. Fluids were designated as low (< 60 mEq/liter; 50 fluids) or high (> 60 mEq/liter; 106 fluids) sodium fluids. Transmission electron microscopy identified differences in the depths of apical tight junctions between cells from cyst walls of 12 of the low and 10 of the high sodium fluids from two kidneys (mean +/- SE depths of 2039 +/- 74 A vs. 386 +/- 18 A respectively; P < 0.0001). When fluids were grouped by kidney of origin, six of the 15 kidneys had only high sodium fluids. The probability that chance had led to the sampling of only high sodium fluids in these organs, given that 32% of all fluids were low sodium fluids, was calculated at < 0.00015. The possibility must be considered that all kidneys are not alike in ADPKD.