The current proposal describes a post-market, clinical registry of HydroSoft, aimed at gaining robust clinical data in a large set of patients to better define the advantages, and potentially, the disadvantages of the HydroSoft, and to inform future randomized trials.

Aim 1. (Primary Outcome) To test the hypothesis that HydroSoft coils lead to diminished rates of aneurysm recurrence relative to bare platinum coils. In order to test this hypothesis, we will compare recanalization or recurrence rates observed in this prospective registry of HydroSoft coils used as finishing coils to historical series of aneurysms treated with bare platinum coils. This registry is designed to achieve 80% power to detect a minimally clinically relevant difference in rates of recurrence between HydroSoft-treated aneurysms compared with bare platinum coils. Based on recent, prospective, randomized trials assessing both bare platinum coils and hydrogel-bearing coils with 25% and 15% recurrence rates, respectively, we predict a recurrence rate of 18% for HydroSoft treated aneurysms. Typically, a "minimally clinically relevant difference" is on the order of a 30% diminution in a given undesirable outcome. As such, the registry is well powered to detect this minimally clinically relevant difference (18% expected recurrence rate for HydroSoft treated coils versus 25% for the historical control group).

Aim 2. (Secondary Outcome 1) To test the hypothesis that the HydroSoft embolic coil can be placed reliably in intracranial, saccular aneurysms, even late in the embolization procedure. Success of this aim is defined as ease of placement of the HydroSoft device, without need for coil removal and subsequent "finishing" with a bare platinum coil.

Aim 3. (Secondary Outcome 2) To test the hypothesis that use of the HydroSoft embolic coil is associated with risks of thromboembolism similar to that with other coil types, on the order of 3-5% or less.

Inclusion Criteria:

1. Patient presenting with a ruptured or un-ruptured cerebral aneurysm appropriate for endovascular treatment as determined by the neurovascular team (neurosurgeon/neurointerventionist)
2. Patient age: patients aged between 21 and 90 years will be eligible.
3. Patient HUNT AND HESS Grade 0-3.
4. Patient has given fully informed consent to endovascular coiling procedure or if patient cannot consent for themselves, appropriate written consent has been sought from their next of kin, or from appropriate power of attorney.
5. Aneurysm 3-15mm in maximum diameter.
6. Patient is willing and able to return for follow up angiography or MRA between 6-18 months.
7. The patient has not been previously entered into this registry
8. The aneurysm has not previously been treated (by coiling or clipping).

Exclusion Criteria:

1. Inability to obtain informed consent.
2. Medical or surgical comorbidity such that the patient's life expectancy is less than 1 year.

All people with cerebral aneurysms between the ages of 21 and 90 that are treated with coils.