Urine methylmalonic acid (MMA) concentrations were detected in 79 Chinese patients by gas chromatography mass spectrometry (GC/MS), using a selected ion monitoring program. 10 of the 79 patients were found to have cobalamin deficiency. Their urine MMA amounts were all elevated with a mean value 1376 ng/microliter (11.66 mmol/l), ranging from 40.46 to 3900 ng/microliter (0.34-33.05 mmol/l). The remaining 69 cases were found to be unrelated to cobalamin deficiency. Their mean urine MMA was 3.62 ng/microliter (30.0 mumol/l), ranging from 0-17.47 ng/microliter (0-148.0 mumol/l). In this study, we found that urine MMA detected by GC/MS was a simple, rapid, convenient, specific and sensitive method for the diagnosis of cobalamin deficiency. The urine MMA concentrations in cases not due to cobalamin deficiency would not exceed 20 ng/microliter (169.5 mumol/l), whereas in cobalamin deficiency the urine MMA levels always exceeded 20 ng/microliter, or were even much higher. No overlapping of the results of urine MMA between these 2 groups of patients could be seen in our study. Detection of urine MMA is useful in the demonstration or exclusion of cobalamin deficiency in any suspect patients.