Occurrence of Legionella pneumophila can be relevant to the installation age and the presence of heterotrophic plate counts (HPCs). This research illustrates L. pneumophila contamination of hospital water in accordance with the installation age and the presence of HPCs. One hundred and fifty samples were collected from hot and cold water systems and cultured on R2A and BCYE agar. L. pneumophila identification was done via specific biochemical tests. HPCs and L. pneumophila were detected in 96 and 37.3 % of the samples, respectively. The mean of HPCs density was 947 ± 998 CFU/ml; therefore, 52 % of the samples had higher densities than 500 CFU/ml. High densities of HPCs (>500 CFU/ml) led to colonization of L. pneumophila (≥1000 CFU/ml), mainly observed in cooling systems, gynecological, sonography, and NICU wards. Chi(2) test demonstrated that higher densities (>500 CFU/ml) of HPCs and L. pneumophila contamination in cold water were more frequent than warm water (OR: 2.3 and 1.49, respectively). Univariate regressions implied a significant difference between HPCs density and installation age in positive and negative tests of L. pneumophila (OR = 1.1, p < 0.001, OR = 1.2, p < 0.001). Mann-Whitney U test implied the significant effects of HPCs and installation age on L. pneumophila occurrences (p < 0.001). Spearman correlation and multivariate linear regression revealed significant differences between L. pneumophila and HPCs densities (r s  = 0.33, p < 0.001 and ß = 0.11, p = 0.02), but nonsignificant difference with installation age (r s  = 0.33, p < 0.001 and ß = 0.0, p = 0.91). The occurrence of L. pneumophila, HPCs, and installation age are relevant; so, plumbing system renovation with appropriate materials and promotion of the effective efforts for hospital's water quality assurance is highly recommended.