Discoloration of the leg following vaccination is a relatively unknown entity. We carried out a study of discolored leg syndrome (DLS) during a 10-year consecutive period with the objective of characterizing DLS in infants following vaccination received in the Dutch National Vaccination Program as well as its occurrence and association with different vaccines. Discolored leg syndrome was defined as an even or patchy red, blue or purple discoloration of the leg(s) and/or leg petechiae with or without swelling. All reports of adverse events following immunization that were made to the passive surveillance system between 1994 and 2003 were included-a total of 1162 identified cases. Red, blue, purple discoloration and isolated petechiae were reported in 39, 19, 27 and 14% of these cases, respectively. Of these 1162 cases, 1105 were considered to be related to the vaccination, based on a predefined risk window with symptom onset after vaccination (48 h for discolorations and 2 weeks for petechiae). Of the 1105 cases, about 50% occurred after DTP-IPV+Hib1 vaccinations, and 30% occurred after DTP-IPV+Hib2 vaccinations. Discolored leg syndrome was frequently accompanied by fierce crying (78%). The median time interval between vaccination and the occurrence of DLS was 3.8 +/- 46.7 h, and the median duration was short (2 +/- 61.7 h). Advancing the vaccination schedule from 3 to 2 months of age caused a small increase in DLS. Discolored leg syndrome manifested mainly after the first and/or second vaccination. In addition to dose, the occurrence of DLS may be slightly age-dependent and self-limiting. The pathophysiology is unknown but may be the result of a vasomotor reaction. Future studies should elucidate the recurrence rate, identify risk factors and assess late outcomes.