1. Serum serotonin (5HT) was determined in normal and autistic children by high performance liquid chromatography with electrochemical detection (HPLC-ED), after serum deproteinization. 2. The sample deproteinization was carried out by the addition of 3.4 M HCLO4 to a small volume of the serum, followed by freezing, centrifugation, dilution and injection of sample into the HPLC. 3. Serum 5HT concentration was significantly increased in autistic children (303 +/- 92 ng/ml) (N = 19) when compared to that of normal children (215 +/- 67 ng/ml) (N = 46). The data of normal children analyzed by ANOVA for sex or age showed no difference. 4. The method employed in this study showed high resolution, good sensitivity and can be used for routine determination of serum 5HT in the clinical investigation of hyperserotonemia in autism.