This study was prompted by 5 patients (seen by OGR) presenting with unilateral enlargement of the breasts 4 to 9 years following augmentation mammaplasty with saline-filled implants. At exploration one breast implant was seen to be markedly enlarged when compared to the other, with a brownish yellow material that had the consistency of serum, Studies were undertaken to determine the permeability of the silicone container to various body fluids and a study of the contained fluid itself, Protein measurements, viscosity measurements, and osmotic water permeability measurements were performed. The results were consistent with the hypothesis that these silicone implants were indeed permeable to both water, glucose, and protein. We hypothesize that the mechanism underlying this in vivo expansion is colloid osmotic swelling, Why one breast should be more involved than the other is unknown. We believe that this phenomenon is occurring more frequently than is being reported.