Objective: To investigate if the incidence of disorders of spermatogenesis and testicular tissue morphology have changed in middle aged Finnish men over 10 years. Design: Two necropsy series completed in 1981 and in 1991. Setting: Department of Forensic Medicine, University of Helsinki, Finland. Subjects: 528 men, aged 35 to 69 years, subjected to medicolegal necropsy. Main outcome measures: Scoring of spermatogenesis and morphometric analysis of testicular tissue components. Individual risk factors for testicular disorders obtained by postmortem blind interviews with acquaintances. Results: Normal spermatogenesis was found in 41.7% of the men (mean age 53.1 years). Between 1981 and 1991, the ratio of normal spermatogenesis decreased significantly (odds ratio 3.5; 95% confidence interval 2.5 to 5.1) from 56.4% to 26.9%, with a parallel increase in the incidence of partial and complete spermatogenic arrest (2.1; 1.4 to 2.9 and 2.9; 1.7 to 5.0, respectively). During this period, the size of seminiferous tubules decreased, the amount of fibrotic tissue increased, and the weight of testicles decreased significantly. Alterations in testicular characteristics over time could not be explained by changes in body mass index, smoking, alcohol drinking, or exposure to drugs. Conclusions: The incidence of normal spermatogenesis decreased among middle aged Finnish men from 1981 to 1991, and the incidence of disorders of spermatogenesis and pathological alterations in testicles increased. Deteriorating spermatogenesis may thus be one important factor in the explanation of declining sperm counts observed worldwide. Several recent reports have suggested a significant decrease in human sperm counts over the past few decades We used middle aged subjects, with no biasing selection as regards fertility or status of spermatogenesis, to evaluate changes in the incidences of disorders of spermatogenesis from 1981 to 1991 Normal spermatogenesis was observed significantly less often in the 1991 series than in 1981, whereas the incidences of disorders of spermatogenesis increased significantly during that time Changes in the status of spermatogenesis coexisted with decreased testicular weight, smaller seminiferous tubules, and increased fibrosis of testicular tissue Alterations in spermatogenesis could not be explained by a change in individual risk factors between the series, such as smoking, drinking, or use of medication, thus challenging further research to illuminate specific reasons for deteriorating spermatogenesis and declining sperm counts