Arsenic trioxide (As2O3) has been a research focus because of its promising anticancer effects especially in the treatment of leukemia. Another arsenic compound, realgar (As2S2), has long been used as a therapeutic agent to treat some diseases in ancient China and Europe, and its medicinal effects have attracted increasing attentions in recent years. However, its poor water-solubility unfortunately results in poor bioavailability and hampers it from being studied and used for possible clinical application. In this study, nanosized realgar particles were prepared by cryo-grinding with polyvinylpyrrolidone (PVP) and/or sodium dodecyl sulfate (SDS). Major physical properties of the respective nanosized realgar particles were characterized. Co-grinding realgar with PVP and/or SDS produced smaller and more monodisperse suspension of nanoparticles. The in vitro cytotoxic effects of such nanosized realgar particles on selected human ovarian (CI80-13S, OVCAR, OVCAR-3) and cervical (HeLa) cancer cell lines were investigated. Significant anti-proliferation effect of these realgar nanoparticles on these cancer cell lines was observed. CI80-13S was most sensitive to the nanosized realgar particles with IC50 values of less than 1 microM as As2S2, whereas the other cancer cell lines had IC50 values in a range of 2-4 microM as As2S2. The cytotoxic activity of the realgar nanoparticles to these human gynecological cell lines was comparable to arsenic trioxide observed previously. In these cancer cell lines, the cytotoxic effects were caused by apoptosis as confirmed by cell cycle and DNA laddering analysis. In in vivo study, a remarkable increase in urinary recovery of arsenic was observed in rats after a single oral administration of the cryo-ground realgar particle suspension. Ranging from 58.5 to 69.6% of the administered dose of arsenic was recovered in urine in the first 48 h from the PVP and/or SDS co-ground preparations; whereas the original realgar powder gave a urinary recovery of only 24.9%. The finding suggested that size reduction of realgar particles to nano levels could enhance its bioavailability substantially.