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Background: Excretion of creatinine in urine represents the end-point of endogenous energy transfer from stored adenosine triphosphate in skeletal and cardiac muscle. Measurement of urinary creatinine is commonly used to correct for total urine concentration. Various quantitative measures of compounds suspected to be either pathological to, or indicative of, possible therapeutic interventions for Pervasive Developmental Disorders (PDD) have relied extensively on spot creatinine as a ratio quantity, although this important metabolite has not been exclusively studied within this population. Methods: Levels of urinary creatinine in spot urine samples were analyzed for a group of children diagnosed with PDD ( n = 24; median age, 75 months; range, 39–137 months) and a control group ( n = 50; median age, 109 months; range, 59–140 months). Diagnosis of PDD was confirmed using the Autism Diagnostic Interview–Revised. Samples were collected and analyzed blind for creatinine content using an improved Jaffe’s reaction method. Results: Controlling for sample pH and body mass index, a significant decrease in urinary creatinine concentration was found in the PDD group compared to controls using a Mann–Whitney two-tailed ranks test ( P = 0.001). Conclusion: Further studies of protein catabolism and renal function in autism are required to ascertain the relevance of decreased spot urinary creatinine excretion identified in this preliminary study. Issues regarding the use of single urine creatinine measurements and associated confounding variables are discussed in light of the findings, together with recommendations to use other internal or external standards for the quantification of urinary compounds in PDD research. [ABSTRACT FROM AUTHOR]