EL-SAYED, M. S. and B. DAVIES. A physical conditioning program does not alter fibrinogen concentration in young healthy subjects.Med. Sci. Sports Exerc., Vol. 27, No. 4, pp. 485–489, 1995. Twenty five subjects were divided into experimental (N= 13, 7 male and 6 female, age 32.1 ± 6.4 yr) and control groups (N= 12, 6 male and 6 female, age 33.4 ± 5.4 yr) in order to examine the effect of a conditioning program on fibrinogen concentration. Before and after conditioning, &OV0312;O2maxwas determined in all subjects. The experimental group (E) exercised for 30 min, 3 d·wk-1for 12 wk at 70% (initial 6 wk) and 80% (latter 6 wk) of maximum heart rate. The control group (C) maintained normal activity pattern. After conditioning, subjects in E, but not in C, increased (P> 0.001) their &OV0312;O2max. Resting fibrinogen concentrations (RFC) before conditioning were similar between the two groups, and demonstrated no change in the C group (242.9 ± 40.3 mg·dl-1vs 247.4 ± 38.7 mg·dl-1,P> 0.05) after conditioning. Although RFC showed a 6% decrease in the E group post conditioning (249.1 ± 21.5 mg·dl-1vs 236.4 ± 34.6 mg·dl-1), this decrease was not significant (P> 0.05). A significant increase (P< 0.05) in fibrinogen concentrations were observed after maximal exercise in both groups pre and post conditioning; however, this increase disappeared (P> 0.05) when data were corrected for hemoconcentration. These results suggest that after conditioning the 6% decrease in RFC was not statistically significant, but the possibility of Type II error is high.