The PIAMA study evaluates whether application of mite impermeable mattress covers reduces exposure to mite allergen sufficiently to reduce the incidence of asthma and mite allergy. The study started in 1996. Baseline measurements of mite allergen levels were conducted on mattresses of newborn children and their parents, mostly in 1997. Mite allergen levels were surprisingly low in this study compared with previous studies among school children and infants. Mite allergen levels were measured again on mattresses in the PIAMA study in the years 2000/2001 when the children were 4 years old, and in a new study among 6-12 year old school children conducted in the fall of 2001. Data on winter climate were collected as well. In the winters of 1995/1996 and 1996/1997, which preceded and coincided with the PIAMA baseline measurements, temperatures had been extremely low, and precipitation had been extremely low as well. It is likely that these unusual winter weather conditions affected the baseline allergen levels in the PIAMA study so that the effect of the planned intervention (mite impermeable mattress covers) was considerably smaller than it could have been.