Background: Throughout the Western world, response rates are falling in population surveys. The aim of the study was to explore for the presence of non-response bias in two Swedish research projects on HIV [‘HIV in Sweden’ (HIS11)] and sexuality [‘Sex and health’ (UngKAB09)]. Methods: The study used four data sets (two from each project), each of which had been generated using different methods. By comparing means and percentages on 15 items across the data sets, we explore the potential presence of non-response bias due to interest in the survey topic (topic salience), and discuss the suitability of two increasingly common methods for recruiting respondents: the pre-recruited probability web panel and the self-selected web survey. Results: While a higher proportion of the respondents in the HIS11 substudies had been tested for HIV and were inclined to perceive themselves as being at high risk of HIV infection, the respondents in the UngKAB09 substudies were on average more sexually active. Further, while there was little variation in the results between the HIS11 substudies (postal/web survey and pre-recruited web panel), there were some fairly large differences in the results between the UngKAB09 substudies (web surveys, one of which was based on a self-selected sample). Conclusion: The study concludes that (i) there are signs of non-response bias that may be due to topic salience, (ii) while care must be taken when using self-selection sampling methods, the pre-recruited probability web panel might provide a cost- and time-effective alternative for recruiting respondents in future population surveys.