BACKGROUND: According to the single distribution theory advocated by Rose, the prevalence of a deviant condition such as excessive alcohol consumption depends upon the average level of the corresponding characteristic in the population. The objective of this study was to establish whether the single distribution theory applies to gambling behaviour. METHODS: Household gambling expenditure in the United Kingdom was examined using Family Expenditure Survey data collected before and after the introduction of a national lottery in November 1994. RESULTS: In cross-sectional analyses, the mean (or median) household expenditure on gambling for each region predicted the prevalence of excessive gambling in that region: the slope of the relationship in 1995-96 was equivalent to an increase of 1.2 (95% CI 0.7-1.7) points in the percentage of households gambling more than 10% of income for every increase of 1 Pound in mean household gambling expenditure. The introduction of the national lottery was associated with an increase in mean household gambling expenditure from 1.45 Pounds to 3.81 Pounds per week, and an increase in the proportion of households gambling more than 10% of total income four-fold from 0.4% to 1.7%. Among households with income of less than 200 Pounds/week, the proportion gambling more than 10% of their income increased from 0.6% to 3.2%. INTERPRETATION: The single distribution theory applies to gambling behaviour. The increase in average gambling expenditure associated with the introduction of a national lottery in the United Kingdom has led to a pronounced increase in the prevalence of excessive gambling, especially in low-income households. This is likely to increase the prevalence of gambling disorders and to exacerbate social inequalities in health.