The booster effect varies between populations, but has not been studied in the UK. The aim of this study was to investigate the effect of repeat tuberculin tests at 1 wk in BCG-vaccinated healthy subjects (all hospital employees) in the UK; we have assessed whether a booster effect is present 48 and 72 h after injection. Twenty-six subjects received two tuberculin tests (both 10 units) administered by the Mantoux technique-Tests 1 and 2. At Test 2 there was a significant increase in induration at 48 h (mean, 7.8 mm; p < 0.001), no difference at 72 h (mean, 0.2 mm; p = 0.93), and a reduction at 96 h (mean, -4.2 mm; p = 0.02). There were more positive results (> 15 mm induration) at Test 2 compared with Test 1 at 48 h (19 vs. 9, respectively; p = 0.002), but similar numbers at 72 h (11 vs. 10, respectively). These results show that repeat tuberculin testing in this BCG-vaccinated population caused a booster effect that varied with the time of measurement and was maximal at 48 h.