We report high-precision transit photometry for the recently detected planet HD 17156b. Using these new data with previously published transit photometry and radial velocity measurements, we perform a combined analysis based on a Markov Chain Monte Carlo approach. The resulting mass M-p = 3.09(-0.17)(+0.22) M-Jup and radius R-p = 1.23(-0.20)(+0.17) R-Jup for the planet places it at the outer edge of the density distribution of known transiting planets with rho(p) = 1.66(-0.60)(+1.37) rho(Jup). The obtained transit ephemeris is T-Tr = 2 454 438.48271(-0.00057)(+0.00077) + N x 21.21747(- 0.00067)(+0.00070) BJD. The derived plausible tidal circularization time scales for HD 17156b are larger than the age of the host star. The measured high orbital eccentricity e = 0.6719(-0.0063)(+0.0052) can thus not be interpreted as the clear sign of the presence of another body in the system.