To the best of our knowledge, this is the first published investigation of the size and position of chimpanzee spleens as measured by ultrasound, and that a fine-needle biopsy of the chimpanzee spleen, guided by ultrasound, has been performed. In general, the chimpanzee spleen is smaller than the human spleen (mean length of 8.9 cm, SD 1.17 cm; mean thickness 2.4 cm, SD 0.35 cm; n = 14) and its position may be different due to different numbers of thoracic vertebrae. Fine-needle biopsy is a safe and simple method to obtain lymphoid tissue sufficient for PCR testing of HIV-1.