The spleen acts as a major site of clearance of antibody-coated platelets from circulation in immune thrombocytopenia (ITP). Splenectomy carries a high cure rate. The biological effect of a single therapeutic dose of ultrasound directed transthoracically to the spleen at 1 MHz and 1 W/cm2 with a mean treatment time of 5 min as generated by Sonopuls 463 (Enraf Nonius) was studied in 30 children with ITP (20 chronic, 10 acute) aged 8-14 years (median 10) and 10 control children. The chronic ITP cases had platelet counts (PC) of 20-50 ×109/1 (mean 36×109/1), showed peak responses at 4 h after exposure 5-18 × 109/1 (mean 10 × 109/1) in 70% of cases, while the remaining 6 patients showed either no change in PC (n = 3) or a decline in PC (n = 3) 5-7 × 109/1. Children with acute ITP had pretreatment PC of 30-50 × 109/1 (mean 40 × 109/1). All had increments of PC after ultrasonic exposure (10-30 × 109/1; mean 18 × 109/1) peaking at 4 h. Six patients with acute ITP maintained the rise in PC while in an other 4, PC returned to baseline in 24 h. The control group showed no change in PC. This therapy was well tolerated and was not associated with significant change in serum lactate dehydrogenase (LDH) levels except in 2 cases with chronic ITP, in whom the LDH levels doubled. All above results were reproduced when therapy was repeated 2 weeks later. In conclusion, this therapy would seem to be safe and well tolerated at such a dose. The effectiveness, rapidity and low cost of this therapy compared with conventional approaches may suggest its use as an alternative therapy in ITP.