BACKGROUND The prevalence of asthma and other allergic diseases is increasing in many parts of the world. OBJECTIVE To determine the prevalence of asthma, rhinoconjunctivitis and skin allergy symptoms in Morocco as part of the International Study of Asthma and Allergic Diseases in Children (ISAAC). METHODS The survey was conducted using the standardised method of ISAAC Phase III in four centres in Morocco-Casablanca, Marrakech, Ben Slimane and Boulmane-with respectively 1777, 1689, 1008 and 1254 13-14-year-old schoolchildren participating. A comparison of the results with ISAAC Phase I was carried out in two centres. RESULTS The prevalence of self-reported symptoms of wheeze in the last 12 months (6.4-16.2%), nasal symptoms (27.9-52.8%), rhinoconjonctivitis (8.8-28%) and eczema (13.3-20.2%) varied between centres, and were highest in Casablanca, the largest city in Morocco. Significant increases in almost all symptoms were found in the two centres of Casablanca and Marrakech between ISAAC Phase I and Phase III. CONCLUSION Morocco could be classified as a country with an intermediate burden of asthma (between 10% and 15%) and other allergic disorders. The prevalence of these symptoms has increased in the past 5 years.