To evaluate the incidence of diagnosis of Paget's disease of bone in Spain, a retrospective study was performed from 1991-1997 in four Spanish centers to evaluate the number of diagnosed Paget's disease cases. Information about the age, gender, and year of admission and an estimation of the reference population with the number of discharges at the centers were registered for each calendar year. The relative diagnostic risk of Paget's disease was calculated, using 1991 as the reference by Poisson's regression models. The percentage of males among the diagnosed cases varied from 39% in 1992 to 62% in 1993 without significant difference between the different years (p = 031). The great majority of the cases were diagnosed between 50 and 89 yeam of age, from 89% in 1992 and 94% in 1993. During the first years the main part of the cases corresponded to persons younger than 69 years (57.5% in 1991,50% in 1992, and 64.7% in 1993), but from 1994 the major percentage was found in individuals over 70 years of age (68.1% in 1994,53.8% in 1995,623% in 1996, and 58.1% in 1997) (p = 0.045). Once the results were adjusted by the diagnosis center, an increased diagnostic risk of Paget's disease relative to the year of diagnosis was seen. In 1996 the risk was 54% higher than in 1991. This tendency reached statistical significance (p for trend = 0.03). The results of this study demonstrate that there is an increasing diagnostic tendency of Paget's disease along the period of study due to different analyzed variables. (