We have assessed the potential impact of a regular half-day session on Saturday only, or Sunday only, and compared this with a whole weekend on-call service for lung scans. We predicted the effect of these services using the data gathered over 2 years (1992-94), looking at the results of lung scans and admission and discharge of patients. The on-call service in all three cases would be justified if resources from the savings on patient discharge and bed availability could be earmarked for the nuclear medicine service. The cost of introducing such an on-call service for the department would be Pounds 3000 per year per session at the weekend and up to Pounds 10,000 per year for a full weekend on-call service. The total cost to the hospital would be negligible.