Objectives
The aim was to study whether the effects of a population-based health check and lifestyle intervention differed according to study participation rate.

Methods
All persons living in 73 areas of Copenhagen County, Denmark, were included in the Inter99 randomized trial in 1999 (intervention group n = 11,483; control group n = 47,122). All persons in the intervention group were invited for health checks and were offered lifestyle counseling if they were at high risk of ischemic heart disease. Areas were divided into low 35–49%, middle 50–54% and high ≥ 55% health check participation. All persons were followed in registers for 10-year cause-specific mortality.

Results
In high-participation areas, there was a significantly higher risk of lifestyle-(HR 1.37 [1.04, 1.79]) and cancer-related deaths (HR 1.47 [1.08, 2.02]) among women in the intervention group than control group. Regarding smoking-related cancer deaths, differences were even more pronounced. Among men, no significant difference in mortality was seen between control and intervention groups.

Conclusions
The results of this paper suggest that among women, the health check and lifestyle intervention may increase the risk of lifestyle and cancer-related deaths.