Abstract Background The aim of this study is to establish asthma knowledge of parents of children (0–18 years) with asthma at the outpatient clinic. Methods A translated and adapted a 21 item Likert type 5 point scale questionnaire (Cronbach’s α-coefficient 0.73) was completed by 291 parents of children with asthma. Total asthma knowledge scores were associated with demographic and psychosocial variables. Results Factor analysis resulted in a new reduced 10 item questionnaire (Cronbach’s α-coefficient 0.72). Higher educational level of parents was associated with better asthma knowledge (p < 0.008 and p < 0.003). Parents showed more knowledge (p < 0.001) on non-medication questions. Asthma knowledge of the parent did not correlate with child age, gender, duration of airway problems, time since diagnosis or severity of asthma. Conclusions Education of parents concerning the working mechanism, indications and use of asthma medications are an essential part of asthma education. Asthma education should be repeated frequently to parents of children with long-term airway problems or diagnosed asthma. Special attention must be paid to parents with only high school education or less.