The objective was to assess the use of suboptimal doses (60-149 UI kg(-1) day(-1)) of low molecular weight heparin (LMWH) in the treatment of acute venous thromboembolism (VTE) in actual clinical practice and to evaluate the outcomes compared to standard doses (> or = 150 UI kg(-1) day(-1)). Retrospective analysis of data from a multicenter registry of patients with VTE (RIETE; Registro Informatizado de Enfermedad TromboEmbólica). Patient characteristics, antithrombotic treatments, and 3-month outcomes were analyzed. We studied 12,302 patients with VTE; 10,524 patients were treated initially only with LMWH; 1,547 patients received suboptimal LMWH (mean = 122 UI kg(-1) day(-1)), and 8,977 patients received full-dose LMWH (mean = 191 UI kg(-1) day(-1)). The suboptimal group included significantly more patients with recent major bleeding, weight more than 100 kg, raised creatinine, or deep vein thrombosis. No significant differences in mortality rate (7.7 vs 7.8%), VTE recurrence (2.7 vs 2.3%), or fatal hemorrhage (0.6 vs 0.6%) occurred between the suboptimal and the standard group. Major bleeding episodes occurred more frequently in the patients with pulmonary embolism treated with suboptimal LMWH (4.5 vs 2.4%; p = 0.02). In the multivariate analysis, after adjusting for bleeding risk factors, major hemorrhage was not associated with the heparin dose. Suboptimal doses of LMWH are used in actual clinical practice in a reduced group of patients at an outcome rate not very different to that of standard doses. Bleeding episodes depend more on the patient's characteristics than on the LMWH dose. Randomized trials should be performed to corroborate these results.