Given that regardless of actual exposure levels, high-risk perceptions of electromagnetic fields of non-ionizing radiation (EMF-NIR) may cause health effects, it is important to understand the mechanisms behind perceptions in the general population. The aims of this study were to assess perceptions of both exposure and health-risk among mothers of the INMA (Environment and childhood)-Gipuzkoa child cohort; to explore possible determinants that explain such perceptions; and to evaluate whether providing information on exposure levels has any effect on perceptions. Overall, 387 mothers completed a questionnaire composed of four questions on perceived exposure and perceived health-risk of exposure to extremely low frequency (ELF) and radiofrequency (RF) fields answered on a Likert-type scale from 0 to 10. Later, measurements of ELF and RF fields were conducted in the houses of a subsample of 104 participants. All measured levels were far below the levels established by the European Council recommendation. This was explained in the individual reports sent to the families. After reading the results, mothers completed the aforementioned questionnaire a second time, plus two additional questions regarding the role of public health bodies in risk communication. The association between perceived and measured levels as categorical variables was assessed with a chi-square test. Multiple linear regressions were conducted for each of the questions related to perceived exposure and health-risk perceptions. Wilcoxon signed-rank test was conducted to assess the effect of receiving information. Both exposure and health risk were perceived to be very high for both ELF and RF fields, with mean and medians of 7 on a 10-point scale. Reporting higher perception levels was not associated with higher levels of exposure measured at home. Variables that were repeatedly associated with higher perceptions included: manual social class, not having the feeling of living in a good neighborhood, difficulty getting by financially, not having a television antenna within 600 m, being younger and having fewer devices at home. Providing information on EMF-NIR exposure levels at home did not alter health-risk perceptions, but mean perceived RF exposure decreased significantly (by 0.7 points). Most of the participants claimed to have received no or insufficient information regarding exposure and health-risks of EMF-NIR from public bodies and considered it very important that they should.