Bitcoin (BTC-USD), Ripple (XRP-USD), and most other leading cryptocurrencies are well in the red after the first month of 2018. Concerns about stronger regulatory action pressured the cryptocurrency markets throughout most of January. Only one of the top five cryptocurrencies -- Ethereum (ETH-USD) -- finished the month with a gain.

What were the forces behind January's cryptocurrency price moves, and what financial measure should prospective cryptocurrency investors focus on?

Bitcoin symbols on a binary code background.

Image source: Getty Images.

What the five largest cryptocurrencies did in January

Here's a look at the five largest cryptocurrencies by market capitalization, and how much each has changed over the past 24 hours, as well as during the month of January.

Cryptocurrency Name (Code)

Price in U.S. Dollars

Day's Change

Monthly Change

Bitcoin (BTC-USD)

$10,056.00

(0.7%)

(24.7%)

Ethereum (ETH-USD)

$1,109.10

4.3%

47.4%

Ripple (XRP-USD)

$1.11

(4.6%)

(45.3%)

Bitcoin Cash (BCH-USD)

$1,475.50

(1.6%)

(36%)

Cardano (ADA-USD)

$0.50

(6.1%)

(29.6%)

Data source: investing.com. Prices and daily changes as of Jan. 31, 2018, at 1:45 p.m. EST; prices are rounded to the nearest cent where appropriate.

Most major cryptocurrencies, including bitcoin (BTC-USD), performed poorly in January. Ripple (XRP-USD), which had been driven higher by speculators in the wake of major partnership announcements with American Express, Santander, and MoneyGram International, was the month's biggest loser, down by more than 45%. The only winner in the top five was Ethereum (ETH-USD), which soared to an all-time high in January and regained its position as the second-largest cryptocurrency by market cap.

January was full of regulatory fears

The primary driver of the decline in cryptocurrency prices was a fear of global regulatory scrutiny, particularly in key crypto market South Korea. In mid-January, fears that South Korea and China could crack down on cryptocurrency trading caused $300 billion in cryptocurrency market cap value to evaporate in one week. And while many cryptocurrencies have recovered a bit from the lows, a steady stream of regulatory news items have added to volatility and kept prices depressed.

Ethereum was the big winner of January

Unlike most of its peers, second-largest cryptocurrency Ethereum (ETH-USD) had a great January. Most of the gains took place toward the beginning of the month after reports that Ethereum network transaction volume had doubled and that there had been instances of more than 10 transactions being processed per second.

One important takeaway

If you're not a cryptocurrency investor yet and are thinking about buying one of these (or any other) digital currencies, it's important to recognize how volatile they can be. Pay particular attention to the "monthly change" column in the table above: The least volatile of the top five digital currencies moved by about 25% for the month, and January was actually a relatively quiet month by cryptocurrency standards.

An investment in cryptocurrencies can easily fluctuate by 25%, 50%, or even more in a relatively short period of time, so while there's nothing wrong with speculating with your extra money, don't put any money into cryptocurrencies that you aren't willing and able to lose if things don't go your way.

Matthew Frankel owns shares of American Express. He has no position in any cryptocurrencies mentioned. The Motley Fool recommends American Express. The Motley Fool has no position in any cryptocurrencies mentioned. The Motley Fool has a disclosure policy.