This pilot study evaluated hydroxyapatite (HA) implants (Endobon) into bone with magnetic resonance imaging (MRI). Nineteen patients (median age 57 years; range 18-67 years) have been evaluated. Eight received granulated HA into vertebral bodies after trauma, while 11 received HA blocks into extremity bones after trauma (n = 8) or fibrous dysplasia (n = 2). In a 1.5-T MR scanner, transversal T1-weighted, flash, 2-dimensional images were obtained. Signal intensities were measured in regions of interest (ROI) from the centre and periphery of HA, trabecular bone, fat and air before and after gadolinium (Gd) contrast enhancement, and the signal-to-noise ratio (SNR) was calculated. After Gd application the SNR in HA increased significantly, peripherally (11.7 without vs 18.2 with Gd, P = 0.003) more than in the centre (7.9 without vs 11.9 with Gd, P = 0.007). The SNR of the HA was higher in patients with block implants compared with granulated HA (P > 0.008). After Gd application, granulated HA enhances significantly less than HA blocks. During insertion of HA, the granulated form is compressed more and is therefore more compact than the HA block. This might hinder integration into the bone structure and the blood supply. SNR of the HA margin was higher than centrally, which might be due to granulomatous tissue after trauma and/or operation or beginning marginal integration.