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Given the continuing emphasis on preventive medicine in general practice, there is considerable interest in the relative effectiveness of different ways of inviting patients to attend for screening. Recently, opportunistic methods have been advocated as being particularly useful but these methods often fail to reach a high proportion of the target population. Many patients do not consult and when they do they are not always invited to attend for screening. In this study a computer simulation model has been used to examine the effects of these variables in more detail. The notes of a random sample of 190 patients (97 women, 93 men) aged 30-50 years, registered with one general practitioner, were used to provide data for the model. The simulation model showed that increasing the number of screening appointments available each week has only a small effect on screening rates and that a ceiling is reached when 25 appointments per 1000 patients are available. In contrast, increasing the proportion of eligible consulting patients who are invited has a substantial effect such that it could take nearly 12 years to screen 90% of a target population if only one out of every four patients were invited compared with under four years if three out of every four patients were invited. The results suggest therefore that opportunistic screening methods are unlikely to achieve desired screening rates within acceptable time limits. It is argued that to achieve target levels of screening, practices will need to combine opportunistic methods with more formal methods of invitation.