Objective: To examine the age dependency of the urinary para-aminobenzoic acid (PABA) excretion, and if a delayed PABA excretion can be overcome by advancing intake schedule; and to examine the recovery of PABA in fractionated urinary samples collected during 24 h after single and repeated doses of PABA. Design: Cross-over study with subjects randomized to start with recommended schedule of PABA administration (80 mg at 08:00, 12:00 and 18:00; PABA18) and then an advanced schedule (80 mg at 08:00, 12:00 and 15:00; PABA15) or vice versa. One subgroup of eight subjects collected individual urine specimens for 24 h after a morning dose of 80 mg of PABA, and another subgroup of 10 subjects collected individual urine specimens for 24 h after ingestion of 80 mg of PABA three times at mealtimes. Subjects: Employees and relatives from the Danish Food Administration. Setting: Ninety-nine healthy volunteers (61 females and 38 males) aged 30-91y. Results: Linear regressions for PABA15 and PABA18 demonstrate significantly less recovery with age (PABA15: r(2)=0.1784, P=0.0002; PABA18: r(2)=0.1273, P=0.0019). Linear regression of DeltaPABA (PABA15-PABA18) with age showed the best fit line to be horizontal (slope -0.0066, P=0.89; 95%CI -0.1046, 0.0915)and with a Y-intercept not significantly different from 0 (1.575; 95% Cl -4.176, 7.326). In this population the lower limit for complete 24 h urine collection was 79.2%. After a single dosage of 80 mg PABA 70-85% was recovered after 8 h. Within 16 h after ingestion of 240 mg PABA at recommended hours the lowest acceptable recovery (78.1%) was reached. Conclusion: There is a gradual decline of PABA recovery with age that cannot be overcome by advancing the dosage schedule. Because of a lower delimiting PABA recovery for the elderly, some 24 h collections in this age group will be rejected unjustly (false-negatives). Also, with the currently recommended dosage schedule (PABA taken with the main meals) the risk of false-positive 24 h urine collections prevails. With refinement of the PABA test procedure, ie employing a specific analytical method and age-dependent cut-off values, the test may achieve a higher specificity and sensitivity.