INTRODUCTION: Perthes' disease is a childhood hip disorder which frequently precipitates premature osteoarthritis necessitating joint replacement in young adults. The highest incidence reported worldwide is in Merseyside, UK, where a unique disease register is maintained. OBJECTIVE: To describe the temporal trends in disease incidence in a geographically defined area of Merseyside, and to examine the relationship to area deprivation. DESIGN: Descriptive observational study utilising a regional disease register in Merseyside, UK, 1976-2009. PATIENTS: 1082 children with Perthes' disease (682 from a geographically defined area). OUTCOME: Disease incidence by region, year and deprivation quintile (measured by the Index of Multiple Deprivation 2007, and the Child Well-Being Index 2009). RESULTS: There was a dramatic decline in incidence over the study period in Liverpool, with rates falling from 14.2 to 7.7 cases/10,000 0-14-year-olds (p<0.001). Incidence rates halved in nearby Knowsley (p=0.01) but remained largely static in the more affluent region of Sefton, where the annual incidence remained at around 7.2 cases/10,000 0-14-year-olds (p=0.73). The association with area deprivation is striking, with the most deprived quintiles having over three times the incidence of the most affluent quintiles (11.5 vs 3.8 cases/10,000 0-14-year-olds; p<0.001). Incidence by electoral ward was strongly correlated to ward deprivation score (p<0.001). CONCLUSION: There was a marked decline in disease incidence over the study period, particularly in more deprived areas. The magnitude of the association with deprivation, and the changing incidence, strongly suggest that environmental factor(s) are a major aetiological determinant in Perthes' disease.