Skin prick test (SPT) is usually considered to be a safe procedure, but recently there have been occasional case reports of generalized allergic reactions. This study was performed to delineate the prevalence of, and evaluate possible risk factors for, adverse reactions to SPT in a prospective study. Altogether 5,908 patients aged < or =18 yr from 11 different pediatric settings were included. The adverse reactions were classified into two groups: (1) Generalized allergic reactions (GAR), (2) Vasovagal reactions (VVR). Adverse reactions were observed in 14 out of 5,908 children examined with SPT. Seven of the adverse reactions were GARs and required medication, yielding a 0.12% risk for GAR. Seven of 14 were VVRs, giving the same risk, 0.12%. Identified risk factors for GAR were low age (<1 yr) (RR 6.28) and active eczema (RR 16.98). For VVR, the risk factors were female sex (RR 7.32) and multiple skin pricks performed on a single patient (p < 0.05). We conclude that GARs do occur, albeit rarely, so the need for proper emergency handling should always be acknowledged. The risk factors suggested may help to identify patients who need extra attention.