A non-bioactive implant device can easily be changed to in vitro bioactive with a thin coating of crystalline TiO2. This crystalline coating can be deposited very thin with great step coverage at a low temperature with Atomic Layer Deposition (ALD). An anatase TiO2 coating was built up atomic layer by atomic layer using TiI4 and H2O as precursors in a hot wall furnace. Several hundreds of cycles resulted in a 10-30nm well defined TiO2 of anatase phase on both Si and Ti substrates. These coatings were shown to be bioactive when immersed in simulated body fluid in vitro, as hydroxyapatite (HA) formed on the surface. The surface roughness of the substrates affected the adhesion of the HA. The adhesion was low on the smooth Si but much better on the 100 times rougher Ti. The ALD technique is promising for coating substrates of all shapes with bioactive crystalline TiO2 at a low temperature.