The capture-recapture method was used to estimate the sensitivity of case finding in two national STD surveillance systems: (1) STD registration at municipal health services (STD-MHS); (2) statutory notification by clinicians (NNS). To identify those cases common to both surveillance systems, cases from 1995 were compared using individual identifiers. Estimated sensitivities for syphilis were: STD-MHS 31% (95% CI: 27-35%), NNS 64% (56-71%); and for gonorrhoea: STD-MHS 15% (14-18%), NNS 22% (19-25%). The combined sensitivity of both systems was 76% for syphilis and 34% for gonorrhoea. Differences in the sensitivity of the systems were significant. The NNS was more sensitive than the STD-MHS, and the identification of cases was significantly more sensitive for syphilis than for gonorrhoea. A stratified analysis showed comparable results for the two sexes. Knowledge on the sensitivity of surveillance systems is useful for public health decisions and essential for international comparisons.