Adverse reactions after diphtheria, pertussis, tetanus, polio vaccination at 18 months of age were investigated in three groups: 74 children injected in the deltoid muscle with a 16-mm (5/8-in) needle, 64 in the anterolateral thigh with a 16-mm needle, and 67 in the anterolateral thigh with a 25-mm (1-in) needle. No significant differences in systemic reactions were observed. Severe pain occurred in 30.5% of the groups injected in the thigh compared with only 8.1% of the group injected in the arm (P less than .001). Children vaccinated in the thigh had decreased movement of the extremity significantly more often than those injected in the arm (49.9% v 25.6%, P less than .0005), and two thirds of the former limped for 24 to 48 hours. Redness and swelling were observed more often after injection in the arm than in the thigh (58.1% v 26.7%, P less than .0005). The only effect of changing needle length in the groups injected in the thigh was the occurrence of more redness and swelling in children vaccinated with the 16-mm needle compared with the 25-mm needle. Overall, parents rated more reactions as moderate to severe among children injected in the thigh than among children injected in the arm (64.2% v 37.9%, P less than .001). The deltoid muscle appears to be the preferred site for administration of diphtheria, pertussis, tetanus, polio vaccine at 18 months of age.