An analysis of age of onset in juvenile rheumatoid arthritis was performed in the last 300 children seen in our clinic. There was a peak age of onset in girls at 1 to 3 years. Distribution of age of onset in boys was bimodal with the first peak at 2 years of age and the second at 9 years. There was no accentuation of frequency in either sex in the 10- to 14-year age group. The distribution of age of onset was bimodal in both monarticular and polyarticular onset of disease, but no particular modal age of onset was seen with systemic onset of disease. It is possible that these data reflect that JRA is not a homogenous disease, or that there are age-sex related differences in host susceptibility or pathogenic agents.