Cyclophosphamide pharmacokinetics have been studied in 16 female patients with advanced breast cancer. The group included 7 patients who were greater than 20%, less than or equal to 30% over ideal body weight and 5 patients who were greater than 30% over ideal body weight. Cyclophosphamide plasma elimination half-lives ranged between 152 and 984 min (mean 457 min), the apparent volume of distribution between 19.1 and 62.3 1 (mean 36.1 1), and plasma clearance between 25.9 and 166.6 ml/min (mean 69.5 ml/min). There was a significant positive correlation (r = 0.624, P = 0.010) between body weight and plasma elimination half-life, and a significant negative correlation between body weight and cyclophosphamide clearance when normalized to body surface area (r = 0.578, P = 0.019) or normalized to ideal body weight (r = 0.531, P = 0.0345). The apparent volume of distribution did not correlate with body weight. The results show that cyclophosphamide disposition is altered in patients with increased body weight.