Two new cases of postransfusional purpura diagnosed at the same hospital within the space of six months are described. This finding supports the idea that despite being an infrequent disorder, a substantial rise in PTP cases inside and outside our country, has been recorded. This increase has coincided with a greater interest in platelet immunology and, particularly, in complications associated with blood transfusion. Both cases constitute an example of the clinical epidemiological profile which characterizes the patients suffering from this disorder. The patients are two women aged 74 and 60 years who after 8 and 9 days, respectively, of being transfused with red cells developed a severe thrombocytopenia accompanied by generalized haemorrhagic diathesis. The serological studies performed revealed the presence, in both patients, of an HPA-1a platelet specific antibody. The platelet genotyping enabled us to confirm this specificity after detecting an HPA-1 (a-b+) platelet genotype. The treatment with immunoglobulins at high doses proved to be effective in both cases. The adsorption-elution experiments of the antibody versus HPA-1 (a + b) platelets were positive in the patient with the highest antibody titre (1024). This finding support the most recent hypothesis concerning the pathogenic mechanism of PTP. According to this theory, the antibody, which is detected in the acute phase of the PTP, would not yet have acquired the restricted specificity corresponding to it. This could enable it to react with a structure shared by the HPA-1a positive and HPA-1a negative platelets.