We tested the hypothesis that children who sustain a supracondylar fracture have a greater range of elbow hyperextension than those with a fracture of the distal radius. Three observers made 358 measurements in 183 children (114 boys and 69 girls). There were 119 fractures of the distal radius and 64 supracondylar fractures. Initially, the group with a supracondylar fracture appeared to have extension 1.7 degrees greater than that of the group with fracture of the distal radius. On average, there was a maximum variation of 3 degrees between observers. After allowing for age, gender and observer, there was no significant difference between the groups. Our study had greater than 80% power to detect a difference in hyperextension of 2 degrees at the 5% level with the above observer variability. When age and gender are taken into account, any variation in the amount of hyperextension at the elbow is not sufficient to explain the occurrence of a supracondylar fracture.