Objectives This study tested questionnaires using visual analogue scales (VAS) in a cleaning intervention study and attempted to find a simple way of analyzing the replies to the questionnaires. Methods A VAS questionnaire made up of 26 questions was developed and marked once a week for 28 weeks by the room occupants in 3 buildings. A total of 1248 questionnaires was used in the analysis of the results. A simple model based on the differences between a person's average responses during 2 different periods was used in the analysis. Results No clear effect of the cleaning was found. Several significant correlations between different questions were established. Estimates for the design of future studies are given. Conclusions The VAS questionnaire proved to be feasible for this type of study. It is suggested that each intervention period should last 4 weeks if the questionnaire is used once a week. However, the length of the period also depends on the expected latency of the symptoms, on how long it takes for environmental conditions to be affected by the intervention, and on how quickly conditions return to "normal" during control periods.