Cerebrospinal fluid from 8 patients with infantile spasms (mean age: 6.1 months) was collected before treatment. The concentration of cerebrospinal fluid tryptophan metabolites was analyzed using high-performance liquid chromatography and compared to metabolite concentrations in cerebrospinal fluid from 20 age-matched controls (mean age: 5.8 months). The levels of cerebrospinal fluid serotonin, 5-hydroxyindoleacetic acid, and kynurenine were significantly lower in infantile spasm patients compared to controls (P less than .05). In contrast, the levels of cerebrospinal fluid 3-hydroxykynurenine were significantly higher in infantile spasm patients than in controls (P less than .05). There were no significant differences in the levels of cerebrospinal fluid tryptophan and 5-hydroxytryptophan. Although the study population was small, these findings suggest that the presence of seizures in infantile spasms is associated with a decrease in serotonergic metabolites which, in turn, may indicate a decrease in serotonergic activity, altered clearance of these metabolites, or altered turnover in the direction of 3-hydroxykynurenine. The perturbance caused by increased 3-hydroxykynurenine and decreased kynurenine in the homeostatic balance between these 2 tryptophan metabolites could further contribute to the pathogenesis of infantile spasms.