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The Israel Defence Forces needed to define the correlation between physical activity and asthma.To determine whether combat unit (CU) soldiers are more susceptible to exercise-induced asthma (EIA) than other military units.A follow-up study of recruits with normal clinical and pulmonary function tests over a period of 30 months after having been assigned to CU, maintenance units (MU) or clerical tasks (CT). The participants chosen had already been subjected to additional tests 6 weeks after induction to eliminate any cases of active asthma.Out of 799 subjects, 125 developed asthma during the follow-up. Twenty-one per cent of those in the CU developed asthma against 15% in the MU and 5% in the CT. The relative risks for newly diagnosed asthma were 3.7 for CU/CT (P < 0.001), 2.7 for MU/CT (P < 0.001) and 1.4 for CU/MU (P < 0.05). EIA was observed as the only manifestation of asthma in 32% of the soldiers posted in CU compared to 13 and 11% in MU and CT, respectively.The increased risk of EIA in CU compared to MU and CT may indicate that any one or all the factors associated with CU service conditions could contribute to this increased risk of uncovering the mild cases of asthma, especially EIA, that had been overlooked up to the time of induction into the army.