Interdialytic weight gain is believed to influence predialysis blood pressure. Since interdialytic weight gains vary among treatments for individual patients, blood pressure and weight gain data could be examined to determine how weight variations correlate with differences in blood pressure. Therefore, the quantitative effect on prehemodialysis blood pressure of typical interdialytic weight gains was prospectively studied in 19 nondiabetic patients on chronic hemodialysis. Over a mean of 23.6 treatments (range 17-25), the slope of each patient's prehemodialysis blood pressure versus excess weight (prehemodialysis weight minus baseline dry weight) was determined. The mean slope of the prehemodialysis mean blood pressure/excess weight regression line was 1.2 mm Hg/lb excess weight. No significant correlation was found between individual prehemodialysis blood pressure/excess weight slopes and patient age (r = 0.20), months on dialysis (r = 0.33), dry weight (r = 0.05), or mean excess weight (r = 0.19). Slopes did not differ for 3-day versus 2-day interdialytic intervals, hypertension-treated versus untreated patients, or men versus women. In 5 patients, individual prehemodialysis mean blood pressure/excess weight slopes were significantly greater than 0, averaging 2.4 mm Hg/lb excess weight (vs. 0.8 mm Hg/lb in the remaining patients). These 5 volume-responsive patients did not differ clinically from the 14 volume-resistant patients. The weight gains commonly observed in patients undergoing chronic hemodialysis have only a modest effect on prehemodialysis blood pressure in the majority of patients.