Data were pooled from three controlled and double-blind studies of lithium carbonate involving a total of 48 hospitalized children, and secondary data analyses were conducted. The objective was to assess whether there is a relationship between a child's chronological age and side effects associated with lithium administration. Two dependent measures of side effects were investigated: number of side effects per child and number of episodes of side effects per child. The children were diagnosed as having conduct disorder with a profile of severe aggressive and explosive behavior; their ages ranged from 5.08 to 12.92 yrs (mean, 9.23 yrs). For the entire sample of 48 children, the effect of age on side effects was statistically significant (p = .057); younger children had more side effects than older children. This relationship continued to hold after adjustment for weight, serum lithium levels, optimal dose, and duration of optimal dose.