Declining financial skills are detectable in patients in the year before they develop Alzheimer's, according to US researchers.
The researchers say this could be a useful indicator for doctors supporting people with memory problems.
Previous studies have shown that problems with daily activities often precede the onset of Alzheimer's.
But charities said most people having trouble working out figures should not be alarmed by the study.
The research from the University of Alabama in Birmingham is published in the journal, Neurology.
The researchers studied 87 people with mild cognitive impairment (MCI), 25 of whom developed Alzheimer-type dementia during the study period, and 62 who did not.
They compared them with 76 healthy people with no memory problems.
They used a tool called the Financial Capacity Instrument (FCI) to measure their skills over a period of a year.
The FCI looks at skills including understanding a bank statement, balancing a cheque book, paying bills, preparing bills and counting coins and currency.
Tested at the start of the study and then a year later, the overall FCI scores for the 25 patients who progressed to Alzheimer's disease, showed a 6% decline.
Their skills in managing a cheque book dropped by 9%.
The control group and the 62 people with MCI who did not progress to Alzheimer's maintained the level of their FCI scores throughout the year.
Professor Daniel Marson, who led the study, said: "Declining financial skills are detectable in patients with mild cognitive impairment in the year before their conversion to Alzheimer's disease.
"This indicates that physicians and health-care providers need to watch patients with MCI closely for declining financial skills and advice families and caregivers to take steps to avoid negative financial events."
Dr Susanne Sorenson, head of research at the Alzheimer's Society, agreed that this could be a useful indicator for doctors.
She said: "Everyone struggles now and then to divide a restaurant bill or tot up a cheque book.
"However, this study suggests that if you already experience significant memory problems and start to notice a decline in your financial skills it could be a sign of developing dementia."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said it was important to stress that the study only focussed on people with pre-existing memory conditions.
She said: "People who miscalculate a restaurant bill or add up their change wrong should not necessarily be concerned.
"The findings do suggest that a decrease in specific money skills could indicate a progression towards Alzheimer's, and this is something doctors could take into account when monitoring patients."