BACKGROUND Low-molecular-weight heparins (LMWHs) are being preferred to unfractionated heparin (UFH) because of their superior convenience and a comparable or slightly better toxicity profile. Whether LMWH has an inhibitory effect on aldosterone that causes hyperkalemia is yet uncertain. METHODS Twenty-eight patients (all male; mean age: 70 years, range 52-87 years) placed on LMWH therapy (40 mg subcutaneously every 12 h) for deep venous thrombosis prophylaxis after an operation were included in the study. Transtubular potassium concentration gradient (TTKG) was calculated 1 day prior to LMWH therapy and again after 4 days of treatment. Of the 28 patients enrolled in the study, we were able to calculate the TTKG in only 19 patients: 9 had a urinary osmolarity (either before or after LMWH therapy) less than the serum osmolarity, making the TTKG calculation unreliable. The Wilcoxon signed-rank test was used to analyze differences in the median serum potassium levels and TTKG before and after LMWH therapy. RESULTS All patients had adequate renal function (creatinine clearance >90 ml/min). Mean (+/- SD) serum potassium concentration before LMWH was 4.25 (+/- 0.40) mmol/dl. It increased to 4.35 (+/- 0.41) mmol/dl after initiating LMWH therapy (p = 0.09). Similarly, the mean (+/- SD) TKKG calculated was 5.52 (+/- 2.33) before and 5.97 (+/- 3.06) after 4 days of LMWH (p = 0.54). CONCLUSIONS Unlike UFH, LMWH (Lovenox in doses used for postoperative prophylaxis against deep venous thrombosis does not seem to have a significant effect on potassium homeostasis.