Determining past sea-ice distribution is an important goal of paleoceanographers. Here, we present a possible approach to determining past sea-ice distribution in the Southern Ocean during the Last Glacial Maximum (LGM). Diatoms are the principal opal-forming organisms south of the Antarctic Polar Front; their productivity is partly mediated by the presence/absence of sea ice. We reasoned that there should be good coherence between percentage biogenic opal in surface sediments and percentage annual sea-ice cover. This hypothesis was tested by comparing percentage biogenic opal in surface sediments against modern-day sea-ice cover in surface waters directly above each core site. The chronology for each core was determined by various means (biostratigraphy, 14C age dating, and carbonate and opal stratigraphy). With the resulting curve we estimate that yearly concentration of sea ice can be determined to within 30%. Using these data, we estimated percentage sea-ice cover during the LGM for a number of sediments sites (50-66°S) from the Southern Ocean. Core sites now beneath 100% open water witnessed some 25-60% sea ice during the LGM, while core sites presently beneath sea ice during half of the year witnessed more than 75% sea-ice cover during the LGM.