Most studies of childhood ITP focus on platelet count as the sole outcome measure. Although side effects of drug treatment for ITP are mentioned in some studies, serial quantitative measures of drug toxicity over time have not been measured. We hypothesized that it was possible to develop a valid instrument to quantitate drug side effects in children with ITP for use in future clinical trials and to assist with patient management. Accordingly, we designed a “Childhood ITP Symptom Checklist” for completion by parents of children with ITP daily for 7 days following initiation of therapy. Parents were asked to rate on a 0 to 4 scale (0 = none, 1 = mild, 2 = moderate, 3 = severe, 4 = very severe) the severity of 11 possible side effects (headache, hyperphagia, vomiting, moodiness, fever, insomnia, chills, fatigue, leg/arm pain, abdominal pain, nausea). Seventy-four children (50 male) with ITP (46 newly-diagnosed and 28 established) were enrolled from six centers across North America. Age ranged from 6 mo to 17 yr (median 4 yr). Median platelet count on day 0 (when drug therapy was initiated) was 9,000 per mm3 (range 1 to 75,000 per mm3). Fourteen patients received corticosteroids, 28 intravenous immunoglobulin (IVIG), 20 anti-D immunoglobulin, 2 multiple drugs, and 10 no drug therapy. Parent acceptance of and compliance with the instrument was good. All 11 side effects were noted in patients in each group, but the percentage of patients with side effects and the severity and time course varied greatly. Severe or very severe (S/VS) complications were seen with each drug therapy. IVIG-treated patients had maximal side effects on day 1 including headache (in 54% of patients, S/VS 25%), vomiting (36%, 14% S/VS), fever (50%, 14% S/VS), chills (33%, 7% S/VS), and nausea (41%, 11% S/VS). Side effects diminished on day 2 and were minimal thereafter. Toxicity with anti-D was similar, but the profile differed, being maximal on day 0, continuing throughout the week, but not usually S/VS. Thirty-three percent of children receiving anti-D had headache, 38% fever (worse on day 1), 25% chills, and 22% nausea. The side effects were different with corticosteroids. Hyperphagia occurred throughout the entire 7 days in at least 50% of patients and was sometimes S/VS. Moodiness occurred daily and was S/VS one-third of the time. Moodiness and insomnia as well as fatigue were also commonly reported in other patient groups (including no drug therapy). Mild arm/leg pain and abdominal pain were seen with all treatments but was more persistent with corticosteroids. Mild to moderate headache, fever, and insomnia was noted in 10–30% of patients irrespective of therapy received. We conclude that the nature, timing, and severity of side effects reported by parents of children with ITP can be scored using an instrument such as the one employed here. Further research should determine the specific effect of drug dose and treatment schedule, relationships among different side effects, and the impact of these toxicities on the child’s and family’s quality of life.