Background: ICOS (inducible T-cell co-stimulator) has a unique ligand (ICOSL). Together they contribute to both antitumor responses/immune stimulation and potentiating immunosuppression. The interaction of ICOS and ICOSL can stimulate CD4 and CD8 ICOS-positive T-cell populations, but also can increase immunosuppressive regulator T-cells. ICOS agonists and antagonists are both under development as novel therapies. We aimed to quantify ICOS and ICOSL expression relationships with tumor type and other clinically relevant checkpoint inhibitors along with evaluating relationships between ICOS and ICOSL expression patterns. Methods: Data from 514 patients with advanced solid tumors at the University of California San Diego were used in the analysis. RNA expression levels of ICOS, ICOS ligand, PD-1, PD-L1, LAG-3, and CTLA-4 from tumor specimens were analyzed at OmniSeq. Tumor mutational burden (TMB) was also evaluated. Univariate analyses (Fischer’s exact tests) and multivariate analysis (logistic regression) were conducted in SAS v 9.4 to determine relationships between high (>=75 percentile RNA rank) and low (0-24 percentile RNA rank) ICOS and ICOSL expression with age, gender, cancer type, high LAG-3, high TMB, high CTLA4, high PD-1, and high PD-L1. The relationship between ICOS and ICOSL high or low expression were also assessed. Results: Seventy patients had high ICOS (14%) and 192 had high ICOSL (37%) while low ICOS was seen in 226 (44%) and low ICOSL was seen in 76 (15%). High ICOS RNA levels were independently associated with patients not having colorectal cancer (p=0.0009), tumors with high PD-1 expression (p=0.025), tumors with high PD-L1 expression (p<0.0001), and tumors with high CTLA-4 expression (p<0.0001). Low ICOS RNA levels were independently associated with patients whose tumors did not have high PD-1 (p=0.0006), tumors that did not have high PD-L1 (p=0.01), tumors that did not have high CTLA-4 expression (p<0.0001), and those with low ICOSL expression (p=0.0007). High ICOSL was independently associated with a diagnosis of colorectal cancer (p=0.0015), tumors that did not have high PD-L1 expression (p=0.003), and tumors with high CTLA4 (p=0.0001). Low ICOSL was independently associated with patients who did not have colorectal cancer (p=0.0001) and tumors with low ICOS expression (p=0.0032). Conclusions: ICOS and ICOSL RNA expression patterns can provide insight into the complicated ICOS/ICOSL immune checkpoint signaling pathway and help support rational immunotherapy combinations as part of a precision medicine approach. Both stimulators and inhibitors of the ICOS/ICOSL axis are being developed, and their proper application may require interrogation of the individual patient’s tumor/environment.