Platelets from subjects with hyperlipoproteinemia (HLP) differ from normal platelets in lipid composition and function depending upon the phenotypic classification of the HLP. The present study has evaluated the deformability of platelets from human subjects with type IIa and type IV HLP. Platelets suspended in autologous plasma diluted 30-fold with buffer were aspirated into micropipettes 0.7-0.8 microns in diameter by step-wise increment in tension, and the resulting extension lengths were recorded. Platelets from type IIa subjects could not be aspirated as far into the micropipettes as normal platelets. However, less tension was required to reach maximum cell extension than with normal platelets, and the initial extension lengths and slopes of the stress responses were the same as the control. In contrast, platelets from subjects with type IV HLP showed a generalized increase in deformability. The initial cell extensions aspirated from type IV platelets were longer than normal, and larger maximum cell extensions were achieved at lower tensions than control platelets. The type IV platelets were also mechanically fragile and fragmented at lower tensions than control or type IIa platelets. The variance in platelet deformability between subjects of the same phenotype was not directly correlated to plasma lipid or lipoprotein concentrations. This study confirms alterations in the structural organization of platelets from subjects with type IIa and type IV HLP.