A 47-year-old man was referred to our hospital due to a splenic artery aneurysm. The course of the disease was monitored by ultrasound every 6 months. At each examination, the tumor was found to have increased in size by several millimeters, but the sonographer decided that the increase was within the range of error. After 4 years, the tumor showed a rapid increase in size. With a diagnosis of threatened rupture, the patient’s life was saved by surgical intervention. In this case, the observations were only compared to the findings from the previous examination, and the slight increase in tumor size was considered within the range of error. It is important to compare the findings not only with those from the previous examination but also with earlier results.