Seven out of 10 men don't know their partner's bra size, according to new research.
And one in five (19 per cent) admit they couldn't even tell you the colour of their wife or girlfriend's eyes.
But as many as two out of three women (62 per cent) don't tell their partner how much they earn.
The research by fashion retailer Simply Be reveals just how little many men know about the women in their lives.
Despite Valentine's Day being a popular time to give lingerie gifts, the findings showed that 71 per cent of men don't know their partner's bra size, while 74 per cent didn't know their partner's weight.
Husbands and boyfriends fared better when it came to dress size, with 77 per cent getting it right, although less than a third (31 per cent) knew their other half's shoe size.
And it seems there are still some women who like to keep their age a mystery, with 15 per cent of men saying they didn't know what year their partner was born, although the vast majority (96 per cent) did know the day and month of her birthday.
The official job title of their wife or girlfriend is unknown to more than one in five men (22 per cent), but it's the subject of personal finances that really reveals the knowledge gap in many relationships.
Almost nine out of 10 men (89 per cent) don't know what their female partner spends on clothes and shoes.
Things are a little more open when it comes to debt and savings with 71 per cent of men saying they are aware of any debts their partners have, including overdrafts and credit cards, and more than half say they know the value of any of her savings.
Laura Costin, of Simply Be, said: "I would definitely advise any man thinking of buying his wife or girlfriend a lingerie gift for Valentine's Day, to check the sizes in her underwear drawer before hitting the stores.
"Gorgeous underwear is a lovely romantic present - but only if it fits.
"We were not surprised at the number of men that didn't know their partner's weight.
"Hopefully it is because people are less concerned with checking the scales and more interested in keeping fit and healthy.
"Age is another touchy subject for some people, but the way many couples appear to keep their finances separate, or even secret, from their partners was an eye-opener.
"It seems, even in today's very open society, there are still some things we like to keep to ourselves."