Promotion of physical activity is important for community health; thus, it is essential to foment it during the preschool stage. Swimming is one of the common choices, but research assessing its effects on children's physical condition is scarce. This study aimed at describing the changes in physical condition of a group of preschoolers (n=47; age range= 5-6 years), after performing a swimming program. Two non-randomly-assigned groups of preschool children were formed. The experimental group (n=24; 29.2% girls) took part in a swimming initiation program, carried in two 40-minute sessions per week during 10 months. The control group (n=23; 34.8% girls) continued its usual activity. Weight, height, Body-Mass Index, cardiorespiratory capacity («Three minute run»), explosive strength («Horizontal jump»), velocity (20 m), agility («Shuttle-run»), and flexibility («V-sit») were assessed. Data from a total of 47 children was analyzed. Significant changes were observed in cardiorespiratory capacity (p=0.04) and strength (p=0.031) in the experimental group, as well as in agility (p=0.022) and flexibility (p<0.001) in the control group. Taking part in a swimming program significantly improved the cardiorespiratory capacity and strength of the lower body in preschool children. Further research employing randomized controlled trials is needed with the objective of monitoring children's development level, as well as the physical activity they perform in their free time and therefore confirm these results.