The feasibility of using urine samples for the identification of patients with Gaucher disease and carriers has been investigated. It was found that the pH of a urine sample should be pH 6.0 or lower to ensure stability of lysosomal hydrolases. Two parameters of glucocerebrosidase, which is deficient in Gaucher disease, were studied using urine samples from control subjects, obligate carriers and patients. Firstly, the relative level of glucocerebrosidase activity was measured by relating the activity of the enzyme to that of another lysosomal hydrolase. Secondly, the enzymic activity of glucocerebrosidase per unit of protein was measured using an immunological method. The first method allowed discrimination of nearly all obligate carriers of type 1 Gaucher disease from normal individuals. The second method allowed clear discrimination of the majority of carriers from normal individuals, but some obligate carriers were not distinguishable from normal subjects on the basis of this parameter. However, the combination of both methods allowed discrimination between all obligate carriers examined so far (n = 34) and controls (n = 86). There was variability between healthy individuals in the relative amount of glucocerebrosidase in urine samples. A small proportion of healthy individuals have a relatively high activity of glucocerebrosidase in urine samples, reminiscent of observations made in white blood cells by other investigators. In urine samples from two unrelated parents of Gaucher disease patients a level of glucocerebrosidase activity was present that could not be distinguished from that in samples of patients. These individuals represent cases with subclinical manifestation of Gaucher disease, illustrating once more the remarkable heterogeneity in clinical expression of this disorder.