Printed in Great Britain Seasonal Variation in the Incidence of

The seasonal variation in the incidence of 78 consecutive cases of farmer's lung was investigated according to a statistical method described by Roger. The cyclic trend was statistically highly significant (P < 0.0001) and the centre of gravity of incidence appeared in the latter half of April. The reasons for the variation are discussed. Farmer's lung (FL) belongs to a greater entity of allergic diseases of the gas-exchanging parts of the lungs, known as extrinsic allergic alveolitis.1 The allergens of FL are the spores or mycelial fragments of fungi and actinomycetes growing abundantly in hay stored damp. Thus, it is a rather common disease among dairy farmers in areas with a high rainfall.2 A clear seasonal variation has been noticed in the incidence of FL.3"4 The present study deals with more thorough statistical investigation of this cyclic trend. MATERIAL The material consists of all clinically proven new cases of FL treated in 5 Finnish chest hospitals during 1973—1975. The diagnoses and the ad-mission dates were checked from clinical records retrospectively. The number of cases was 78, 19 men and 59 women. The mean age was 43.6 years. METHODS The method proposed by Roger was used to estimate whether there was statistically significant cyclic trend. According to Roger, who has based his method on Edwards ' test for cyclic trends,7 a circle is divided into 12 equal sectors corresponding to months. The monthly frequencies Nj (i = 1,..., 12 according to months) are represented by points of coordinates (x,, yj) placed in the corresponding sectors in the circle. The coordinates (XJ, y,) are computed as follows