HISTORY AND ADMISSION FINDINGS A 24-year-old man with thrombocytopenia was referred for surgical resection of a bleeding polyp of the sigmoid. Examination showed a small haematoma and petechiae on both lower legs. The patient reported that several male family members also had a thrombocytopenic bleeding tendency. INVESTIGATIONS Laboratory tests revealed thrombocytopenia (4000 platelets/ml, with small platelets: mean platelet volume [MPV] 5.6 ml). Serum immunoglobulins were normal. A mutation in the Wiskott-Aldrich (W-A) protein gene (intron 7 + 5 G-->A) was demonstrated both in the patient and his 26-year-old brother. DIAGNOSIS, TREATMENT AND COURSE The diagnosis of W-A syndrome was made and, with perioperative administration of platelets, the polyp was resected without complication. CONCLUSION Most patients with the W-A syndrome die by the time they are aged 10 years, unless appropriate treatment is given. This patient and his brother had a mutation of the W-A protein gene that unusually was in an intron rather than in an exon. Structurally normal W-A proteins were still being formed. This may explain the mild course and late onset of the disease.