Paired testicular volumes and weights, as well as age, height, and weight, were recorded from a series of 1056 consecutive necropsies on adult males ranging in age from 18 to 96 years. These data were analyzed to examine the effects of age, nutritional state (standardized body weight), and illness on testicular size. Testicular volume and weight were related by a constant density of 1.038 g/ml, regardless of testicular size, age or illness. Mean testicular volume was correlated with height (r = 0.470), weight (r = 0.504), body surface area (r = 0.549) and standardized body weight (r = 0.152). Advancing age, malnutrition, alcoholism, malignancy, and a chronic, terminal illness were each individual risk factors for reduced testicular size, whereas diabetes, narcotic or other drug usage, and pelvic injury were not associated with reduced testicular volume. Since advancing age, reduced standardized body weight, and some disease states were all associated with diminution of testicular size, the interaction of age, malnutrition, and illness on testicular size were examined by statistical modeling, using multivariate logistic regression and covariance analysis. The associations of alcoholism, malignancy, and chronic, terminal illness with decreased testicular volume were independent of aging or nutritional state. The effects of chronic, terminal illness were mostly explained by the concurrent effects of reductions in standardized body weight (malnutrition). After exclusion of men with diseases shown to be associated with decreased testicular size, he specific effects of age alone demonstrated a reduction in testicular volume only in the 8th decade of life.(ABSTRACT TRUNCATED AT 250 WORDS)