To investigate the nature and origin of postmastectomy seroma, the concentration of IgG, the number of leucocytes, granulocytes and lymphocytes in the drain fluid and seroma aspirates was studied in breast cancer patients after mastectomy. The composition of the fluid and aspirates and the time-related changes of the investigated criteria suggested that 1) seroma is not an accumulation of serum, but an exudate, 2) the exudate is an element in an acute inflammatory reaction, i.e. the first phase of wound repair, and 3) seroma formation reflects an increased intensity and a prolongation of this phase. By paying attention to factors influencing the duration and intensity of the first phase of wound repair, it should be possible to reduce the occurrence of seroma.