The article builds a mathematical model of preventive maintenance of servicemen using a semi-Markov random process. The exponential law of distribution of time between diseases is chosen as the morbidity model of servicemen. The application of such a law is rightful for servicemen not older than 45 years. A working capacity coefficient is chosen as the criterion for the effectiveness of preventive services for servicemen, and which represents the probability that a serviceman is in working condition at any time. The paper establishes the analytical dependence of the working capacity coefficient, which is proposed, on the parameters of the health care system, namely: the level of morbidity, frequency of dispensary examinations, the probability of applying for medical care in case of illness, the quality of diagnosis of the disease in the sanitary unit and the hospital, the duration of full recovery. The obtained dependences are corresponded to the physical meaning and can be used in practice. The article shows that the working capacity coefficient depends on the frequency of dispensary examinations at different levels of morbidity and on the other fixed parameters of medical care.