One of the authors' young patients suffered near-drowning while being treated with a synthetic below-knee cast, and this intrigued them into examining the buoyancy of waterproof casts and their use in small children. Synthetic and fiberglass casts were fabricated and subjected to immersion testing using lead pellets. Fiberglass casts sank immediately, while synthetic casts stayed afloat. With the addition of weights averaging 221 g, the synthetic casts finally sank. When supine, the casts stayed afloat with more weights. This study shows that synthetic liners are buoyant. Although the weight necessary to sink the casts was small, the weights were lead pellets, which is different from a human limb. Enhanced buoyancy of synthetic casts may pose a potential danger to a small child when not properly supervised in water. The initial buoyant force may be too startling and may place them at a higher risk for near-drowning.