Large-diameter metal-metal total hip replacement (THR) offers the advantages of low wear and low dislocation risk. the aim of this investigation was to compare metal levels in large and small-diameter metal-metal hip replacements. Whole blood concentrations and daily output of cobalt and chromium in 28 patients with unilateral large diameter (42- to 54-mm) metal on metal hip replacements at 1-year follow-up were compared with levels in patients with 28-mm metal-on-metal THRs. Both bearings were made of high-carbon cobalt-chrome alloy. The larger bearing is as-cast and the smaller wrought alloy. High-resolution inductively coupled plasma mass spectrometry was used for analysis. The patients had either a cemented polished tapered stainless steel stem or a cementless porous ingrowth titanium alloy stem. Mean whole blood levels in the small- and large-diameter THRs are not significantly different at 1 year (cobalt, 1.7 vs 2.3 microg/L and chromium 1.7 vs 1.4 microg/L). Daily urinary output of cobalt and chromium was also in the same range and without a significant mean difference (cobalt 11.6 microg/24 h in large-diameter and 12.3 microg/24 h in small-diameter THRs and chromium 3.7 and 4.1 microg/24 h, respectively).