There is no consensus on the best treatment for anterior cruciate ligament hypoplasia or aplasia. To our knowledge, no comparative study between operative and conservative treatment of this condition has ever been performed. Conservative treatment is a viable alternative to surgery for ACL aplasia. Two siblings were examined at our outpatient clinic. The male patient underwent bilateral ACL reconstruction, while his sister was treated conservatively. Our results show a worse long-term outcome for the operative patient. At her last follow-up, the female patient treated conservatively showed subjective improvement in stability and gait. A review of the literature shows inconsistent outcomes after reconstruction in contrast to reports with cruciate ligament agenesis that did not undergo reconstruction with acceptable to good outcomes. Cruciate reconstruction should be reserved for cases of impaired articular instability, objectively manifest in the frequency of giving-way episodes. Treatment depends on the patient's condition and expectations. Surgery should therefore only be suggested after proper patient counseling.