The storage of potato tubers at low temperatures leads to the accumulation of sugars in a process called "low-temperature sweetening." To understand this phenomenon, we measured the sugar contents and the activity of acid invertase over several months in tubers of six Japanese cultivars stored at 4 degrees C or 20 degrees C. At 20 degrees C, few changes in sugar contents took place in any of the tubers. On the other hand, when stored at 4 degrees C, three types of changes were observed among the cultivars: (1) increased levels of reducing sugars during storage; (2) a pattern similar to that of type 1, but with 4- to 6-fold lower levels of reducing sugars throughout storage; and (3) increased sucrose, but not reducing sugars. The activity of vacuolar acid invertase increased in the type-1 cultivars, whereas, in the type-2 and type-3 cultivars, the activities were very low during storage at 4 degrees C. Reverse transcription-polymerase chain reaction analysis of acid invertase showed that the transcripts of the enzyme accumulated in the tubers stored at 4 degrees C in the type-1 cultivars but not in type-3. These results suggest that the activity of vacuolar acid invertase is related to the types of changes that occurred in sugar content during low-temperature storage among the potato cultivars.