The results of a controlled trial of multiphasic screening in general practice are presented. In 1967, 7,229 individuals aged between 40 and 64 years were randomly allocated into either a Screening or Control group. The Screening group were invited to attend two screening sessions held about two years apart, while the Control group continued to receive conventional medical care. Both groups were then invited to undergo a health survey in 1972-73 which revealed no significant differences in morbidity between the two groups. Careful follow-up permitted detailed Screening-Control comparisons of various outcome measures--consultation and hospital admission rates, certified sickness absence from work, and mortality. Nine years after the initial screening, no significant differences were found between the two groups in any of the outcome measures. It is estimated that a similar screening programme for the entire middle-aged UK population would cost 142 pounds million at 1976 prices.