The objective was to identify age-related differences in analgesia after 8 and 16 mg morphine i.m. Retrospective analyses of controlled relative analgesic potency assays in 947 postoperative cancer patients revealed differences among age groups (18-29, 30-49, 50-69 and 70-89 yr) in total pain relief and duration of relief with little differences in peak relief. Weight and initial pain intensity were in the same range among age and dose groups. Aging was associated with enhanced analgesia. The difference in total relief between the extremes of adult age was approximately twice that with twice the dose. While in 50% of the oldest group relief was no longer obtained at 5 h, in 50% of the youngest group relief was no longer obtained at 3 h. These observations suggest that age is a factor in morphine analgesia.