Data from a survey of 1699 currently (once) married males are used in examining ideal family size in Northern Ireland, an area where fertility remains quite high by west European standards. Although there are several problems concerning the measurement and use of ideal family size, an analysis of the Northern Irish data indicates that the responses appear to be valid. For example, most respondents can formulate an ideal family size in numerical terms, there is a considerable degree of variation in the responses and these are not necessarily a function of actual family size. Northern Irish family size ideals (mean 3·6) are considerably greater than those of many west European countries and these ideals also vary within Northern Ireland. Religious grouping proves to be the major factor influencing these variations, with Roman Catholics generally expressing an ideal (mean 4·4) for large families. However, other factors are also associated with variations in ideal family size and the paper considers the roles played by education, age, occupation, region of residence and religiosity. Also, ideal family size does not always correspond with actual family size, indicating that while some married men possess relatively large family size ideals but have smaller families, others have large families but favour more moderately sized ideals. It is evident that attitudes to fertility in Northern Ireland are in a state of change. The value of ideal family size data in understanding fertility patterns is emphasized.