The case-cohort study design, used to reduce costs in large cohort studies, involves a random sample of the entire cohort, called the subcohort, augmented with subjects having the disease of interest but not in the subcohort sample. When several diseases are of interest, multiple case-cohort studies may be conducted using the same subcohort, with each disease analysed separately, ignoring the additional exposure measurements collected on subjects with the other diseases. This is not an efficient use of the data, and in this paper we propose more efficient estimators. We consider both joint and separate analyses for the multiple diseases. We propose an estimating equation approach with a new weight function, and we establish the consistency and asymptotic normality of the resulting estimator. Simulation studies show that the proposed methods using all available information lead to gains in efficiency. We apply our proposed method to data from the Busselton Health Study.