['The objective of the present study was to determine the effect of low-power laser irradiation (LPLI) on cyclic guanosine monophosphate (cGMP) produced by human corpus cavernosum smooth muscle cells (HCC SMC) in vitro.', 'Numerous reports suggest that LPLI is capable of affecting cellular processes in the absence of significant thermal effect', 'HCC SMC cultures were irradiated with single-dose LPLI using a He-Ne continuous wave laser (632 nm) with different energy densities (0.52-2.1 J/cm2). Assessment of effect on cell viability was performed utilizing Alamar Blue assay. Effect of LPLI on cGMP production was studied by radioimmunoassay.', 'We observed that: (1) LPLI of HCC SMC results in a statistically significant increase of cGMP synthesis in culture and is dose dependent (maximal effect was observed with LPLI irradiance of 1.57 J/cm2. (2) There were no changes in Normalized Viability Index (NVI) immediately after and 24 h following laser irradiation.'] Does low-power laser irradiation increase cyclic GMP synthesis in penile smooth muscle cells in vitro?