To test the safety of infusing cryopreserved and thawed peripheral blood stem cells (PBSC) in children, toxicity assessment was made at 36 infusions in 33 children with various types of cancer. The mean volume of PBSC graft infused was 224 ml (range, 46-500 ml) or 8.0 ml/kg (1.7-23.8 ml/kg), in containing 10% dimethylsulfoxide (DMSO). Vomiting was the only toxic feature related to the amount of DMSO. However, eight patients developed transient shock, but recovered shortly afterward with or without supportive therapy. Attention should be drawn to increased risk in children receiving thawed blood cell grafts.