A group of patients were studied while taking phenmetrazine and a placebo, the dietary prescription having been kept constant, using the double blind procedure. The same patients were observed while taking each preparation, thus allowing each one to serve as her own control. The rate of loss of weight while taking phenmetrazine was more than twice that observed when a placebo was used. No significant effects upon the blood pressure, pulse rate, hemoglobin, white blood count, urinalysis, or basal metabolic rate were noted. Side effects while taking phenmetrazine were only slightly more frequent than during placebo medication.