Welsh-speaking parents who talk to their children in English could be putting the language under threat, say university researchers in Bangor.
The 2001 census saw a rise in those who speak write or read Welsh
A study found many Welsh-speaking parents were not "transmitting" the language to children.
The failure was even more marked when only one parent spoke Welsh.
The research also found using Welsh books and television affected children's success with the language.
Dr Delyth Morris, who led the study at the University of Wales, Bangor, said: "Most parents in the study say they want their children to speak Welsh but their commitment varies.
"If a child is to learn a language they must be exposed to it."
The year-long research found that factors such as the amount of time spent with Welsh speakers - whether they are a parent, grandparents, friends of the family or neighbours - affected successful learning of the language.
Dr Morris added: "It seems that when in the presence of an English-speaking partner, Welsh-speaking parents tend to speak English to their child.
"Those parents who can, only actually speak Welsh to their child when they are alone or in the company of other Welsh speakers."
She added: "We found that many Welsh-speaking parents were not transmitting the language to their children.
"This is particularly the case in families where only one parent speaks Welsh."
Childcare is also important to a child's understanding of Welsh.
Childminders, who are Welsh-speaking, and the education policies of local authorities has an impact, say the researchers.
The study, which is seen as important for forming language policy within Wales, is part of an Economic and Social Research Council (SERC) sponsored-study.
The 2001 Census saw a rise of almost 3% in the proportion of people in Wales who can speak, read or write Welsh.
Last November the Welsh Language Board gave £20,000 to a special helpline to help English-speaking parents understand Welsh homework.
It was launched at Ysgol Gynradd Gymraeg Casnewydd in Newport.