We report a clinically isolated toxoplasma pneumonitis in a child treated by HLA haplo-mismatched BMT. Conditioning consisted of TBI, cytarabine and melphalan. The BM graft was T-depleted and the boy received iv moAb antiLFA1 and antiCD2. The clinical course of pneumonitis was characterised by an early onset (day 28) and a rapidly overwhelming course. Donor and recipient had pre-graft IgG Ab against toxoplasma without IgM. These Abs had completely disappeared from the serum of the patient at the time of pneumonitis. PCR amplification detected the B1 gene of Toxoplasma gondii in the patient's PBMC from day 28.